I would say a pro of upgrading is that you get the latest features available to you by upgrading, and adding additional software becomes a pretty simple process. And you will have Avaya's support on the system.
Cons are that you are forced into purchasing IPOSS... Avaya's support program which is pretty inexpensive and has a lot of upside. But you are required to purchase if you upgrade.
If you have little or no interest in the newer features, and your IP500v2 has space or can be reconfigured to accommodate another VCM32v1, that module (as mentioned above) entitles you to 12 IP Endpoint Licenses. And / or you can swap out your VCM32v2 for a VCM32v1 and get the same results. And if you have space and or the ability to reconfigure to make space you can have up to (2) VCM32v1's which entitles you to (24) IP Endpoint licenses in version 9.1. Once you upgrade above 9.1, the entitled IP Endpoint license benefit is eliminated.