Would you be ok with the structural engineer just adding notes to your drawings without consent? Why risk the legal liability of altering the drawings of a registered professional without their consent? Why not just ask the consultant to oversize the column?

Usually I just email my engineer and ask them to change the column. Unless you're working with a real chintzy engineer, they shouldn't have a problem making a tiny change like that...granted, I've usually worked out design changes like that long before I've asked my engineer for signed and sealed drawings.

Your other option is to wait until CA, and just get on the phone with your engineer and G.C. and talk about the change, then have everyone make a note on the drawings. That minor of a change shouldn't warrant any paperwork unless the columns have already been formed or ordered.

EDIT: I'm assuming this is residential. I'm sure the bureaucracy is much deeper when it comes to commercial work.

Nope.

double nope

If it's simple enough to handle with a note, it's simple enough to ask the consultant to do it themselves.

Triple nope. 

So instead of asking the actual consultant to make the change l, you opted to ask total strangers if it’s kosher to draw on someone else’s work? Kidsthesedays  amiright ?

everyday architect. You are correct about the years in practice, approx 40, but not about the done and caught. We are having an interesting intraoffice discussion about what are acceptable practices and I thought it would be interesting to see some outside opinions. Some of our projects are running into long delays in getting consultants revisions in a timely manner. The delays are very costly. one of our PMs wants to notate as a third party re issues that don’t change the consultants’ essential service. In the example I gave, we could note to keep the structural col at 6x6 and case out to 8”, but might prefer to oversize. 

While I find it amusing to be called out as a novice, or a hack, presumably by someone with far less experience, I’d hoped polite discussion on professional practice would be possible. I do appreciate the sincere thoughts enumerated above by others. Fir what it’s worth,  I tend to agree with the don’t do it school of thought. I also wondered what others were doing as I’m sure we are not the only ones running into long delays by consultants. 

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1. For example, hypothetically the structural engineer specifies a 6x6 column, but the architect wants that column to be say an 8x8 for aesthetic reasons. Call your consultant, and direct them to change.
2. Obviously the architect can show it on his own drawings as 8x8. But experience shows that the builder will typically refer only to the engineer’s page for the structural frame. Experience also demonstrates that builders not reviewing all contract documents do one of two things; redo the work, or get fired.
3. Can the architect add a note on the engineers sheet that says something like, "oversize this column to an 8x8"? You propose to do this how, Bluebeam? 
4. If not, how would you handle that override other than asking the engineer to redo his set? Why would the engineer have to "redo his set"? If you hired them, and they want to work with you again, they'll make the change.
5. The underlying assumption is that if a 6x6 is structurally sufficient, then so is the 8x8. What if your assumption was proven incorrect, you'd be one the hook for a number of things, if the work failed. I suspect that your modification of a set drawings, of a discipline, with potential statutory, and ethical consequences would place your status as a 40 year licensed professional in jeopardy, would it not?

I’m waiting for Rick to come in with a rambling response. 

this is a firm NO, NEVER. From management practice and legal liability there are plenty problems - but the most serious is simply safety.

Just to follow your example - imagine you've instructed the contractor to increase all column sizes and they do that. In a simple building I suppose it's fine (but that's only a guess). But we aren't really qualified to judge that - we're just assuming it's fine because in your opinion it's a small adjustment. It's very easy to imagine situations on larger buildings where a small 'adjustment' would cascade into changing fundamental design conditions.

What if increasing column sizes increases the dead load weight on a floor below, overloading columns there? What if it causes an uneven weight distribution at the foundation and causes differential settlement? What if the stiffer structure changes the natural frequency of the building to match the expected frequency of a seismic event - causing it to collapse suddenly in a minor earthquake?

40 years ago a bridge collapsed at a Kansas City hotel, killing 100+ people. The basic cause was a minor design change on a bolt connection made by a steel sub which wasn't reviewed by the engineer. It was a very small change on a small component which the steel fabricator thought was adequately covered by a quick phone call with one of the engineers, and they didn't follow up with a submittal for review because it would slow things down. Small changes sometimes matter. It's not the architect (or contractor's) place to decide which structural changes are important enough to matter.

turns out I have a few example of possible shitstorms. One that I can recall from last year… I needed to increase the thickness of concrete on deck to meet a particular fire rating for the floor. I had previously asked the structural eng but that request never got put into the construction docs. Anyways, I call them and ask if they are cool with an extra 38mm. I knew the steel and spans would be fine since we had other similar projects with them however, I had no considered the impact on their footings, bracing, and rebar… which would require redesign.  Now, they should have been designed to suit the thicker concrete floor in the first place, but it was missed and I don’t need to throw them to the wolves and risk future projects, so I solved the issue without asking them to redo work. Probably came out cheaper for the client too but it’s not like they would have cared. This is a project to house their sports car collection and helicopter. 

I have another example that did quite some damage but it was a clueless client who made the change. End user is a gov department but the executive overseeing the new build decided that the blast force for their mail room was too low. How they knew this? No idea because they clearly did not know that blast force is exponential when they demanded we increase resistance by x5. The half-million extra in rebar just for a 20’x20’ room was only the tip.