HiCCSU-Student,
First of all, I would recommend that if you are in the process of upgrading, that you consider the cRIO platform. It will be much more reliable, future-proof, and capable. However, I understand if a complete upgrade is not possible.
Regarding your question about needing a 16-bit AO module, are you asking if you must have a 16 bit AO module to go with the AI module? Or are you looking for a 16 bit AO module?
Also, out of curiosity, what will you be using the modules for? Or why are you upgrading? If you are looking to upgrade the AO module, the extra context might help us make suggestions.
Thanks