Try doing a
sudo apt-get install gcc-multilib
I don’t think that installing a 32-bit system changes the architecture of your computer, because your computer will always be a 64-bit machine. Installing the 64-bit version of Ubuntu should only give better multi-core performance. Since your computer is still a 64-bit computer, you probably need a C compiler that will compile on 64-bit machines. Hence the gcc-multilib. I think.