Straight off the bat I worry that the overnight batch processes may "rely" on SELECTs being blocked by UPDATEs for consistent results.

Also everything else I read says Optimistic Locking is great, but beware your old code written for pessimistic locking may not return the different results

The Brent Ozar article builds on Craigs article and shows how you get a similar problem with READ COMMITTED to RCSI. It starts in section three.

You both seem to be telling me there is no significant downside or risk to switching on RCSI. Do you enable it on all your databases?