You don't always need a BA if you have someone who does BA's work :-)

I agree, you don't want to engage all of them, but maybe you want to take a risk-based approach. Find out what are your 3 to 5 most business critical sharepoints and engange them. You can focus on their requirements. 

If you have a test-stage for your upgrade, you could try to replicate production and do the upgrade there. Make sharepoint owners accountable for checking the workflow on the test-stage.

If you fail to gather functional requirements, maybe your test-team's time is better spent in non-functional testing? Performance of the software after the upgrade? Does the recovery/backup still work? Do you need zero-downtime? If so, does it work? How about downgrade in case of emergency?