The answer depends mainly on your 3rd party VST library. 32 Bit plugins work best in FL Studio 32 Bit and 64 Bit plugins work best in FL Studio 64 Bit.
Because of this, we recommend you load projects made in FL Studio 32 Bit and 64 Bit in their respective native versions. Loading a 32 Bit project in FL Studio 64 Bit (and vice versa) before you have updated your plugin library to 64 Bit will result in many bridged VST plugins. Bridging uses more CPU and can reduce stability of FL Studio when plugins behave badly (if they do, please report it to Techsupport).
If you want to switch to FL Studio 64 Bit then you should also update ALL your 3rd party VST library to 64 Bit. If you still use lots of 32 Bit plugins, perhaps 64 Bit versions are not available, then we recommend you use FL Studio 32 Bit with those plugins. Remember, you can use both FL Studio 32 and 64 Bit depending on your project and the plugins it uses.
If you want to make a permanent switch to FL Studio 64 Bit you should update ALL your 3rd party VST plugins to 64 Bit format and make sure you use 64 Bit plugins moving forward with new projects.
Check the FL Studio 32 vs 64 Bit FAQ here.
The following video explains the differences between FL Studio 32 and 64 Bit
Image-Line Support Team