I've said it on this forum several times already. Obviously it's an individual decision. I'd keep the scholarship and see what happens.
Cant blame a coach for trying though! Just because he asks doesn't mean you have to give in. If he offered (and you signed) for 50% just six months ago don't you think he thought you were worth it then? Of course he wants to maximize, who wouldn't? At 50% AND that's the school I want to attend, I'd take my chances.
As far as the P5 4-year scholarships are concerned: that's not a NCAA rule so I don't know how each school and conference will handle it. I'd be surprised if the $$ amount CAN'T be adjusted; that's between each school and the player. I'm no expert so perhaps someone else knows about P5 "adjustments" from year to year and can add some insight.