It is much more common for the reverse to happen - planning to architecture. This typically seems based on the highly competitive process of architecture acceptances versus the relatively standard planning admission process. This was certainly the case at my school, as planning was the alternate acceptance for the 90-95% who didn't get into the arch program, myself included. I was fortunate enough to get a transfer offer to a competing architecture school midway through my undergrad.

I found the planning courses I was taking to be exceptional in theory; much more applicable and straight-forward than most of my arch courses. That said, the student work was almost always reduced to buzz-words with seemingly no 'real' project presented at the final. It becomes obvious pretty quickly when 5 different student groups talk about their "sustainable, green-roof'd, densification, public-oriented, etc.etc.etc.etc." without showing any proof how this would work. At least in arch school the teachers would force you to develop some sort of scheme or wall section. I should note that all my old classmates from that planning program are now essentially drawing suburbs.

Personally I could never go back to true planning. Urban design, probably; that part was super stimulating and interesting. You need to go to a program that is directly involved in theories of the city though, preferably in a major metropolitan area. Otherwise expect to be pushing lots of papers and dealing with neighbours complaining about the colour of the other's fence/windowsill/door.