Are you basing that solely on your reading of the footnote on the upgrade page? Because I'm not sure that reading is correct. The way UA is already handling these, it seems the critical date may well be the date of flight departure, not the date the request for an upgrade is made.

Note also that for purposes of upgrade priority on the standby list, the "request" for an upgrade is made at check-in. So for purposes of that list, the new rule will almost certainly be in effect for all flights beginning April 15.