Is it possible to Globally Change specific Opportunities?

No, you're not missing anything. RE doesn't consider proposals a top level record like Constituent, Gift, Action, etc., so you can't query for or change specific proposals in that way. You'll need to query for constituents who have the proposals you need to change, then export all of their proposals. In an external app like excel or access, pull just the proposals you want to make changes to and import the changes back in.