Now that it's getting close to Thanksgiving, we're running a contest to hear advice you've received that you're most thankful for! This can be any type of advice and the advice with the most reactions will win!
If you decide you do want to swap, you would need to be at least 2-3 mo into your intern year before you go to your prelim year PD to explore options. Just figure out where you match first and see how you like IM as a resident first before going too far down this rabbit hole. Your change of heart may be completely legit, but it just does no good to make a plan until you even know where your starting point is.
Continuing in IM after a prelim is relatively straightforward. There are usually multiple programs with open spots. You;d need to decide early enough such that you can resign your advanced match without penalty. Choice of programs and/or location may be limited. The program you matched to prelim might be able to fit you in.