The specifics of which Modules and Toolkits are compatible with which bitness of LabVIEW can vary, depending on which operating system you're using. More information about that is available in the LabVIEW 32-Bit vs. 64-Bit Development Environments FAQ
Source code developed in one bitness of LabVIEW can be generally opened in the other but will need to be recompiled to run. If the code includes functions not available in the new bitness of LabVIEW, the code will not compile. It will instead have a broken run arrow and will warn you about missing dependencies.