yep. you must be a custom made HC. can't do it if you're an owner or real HC.
for the first three years of my CCM, i was jed york. couldn't change a thing. harbaugh won two SBs and retired. i became a custom HC. i've slowly changed my O scheme to the WCO. i only draft players whose scheme is WCO. no exceptions. i currently have one player who isn't WCO. i thought he was and drafted him after a couple of seasons, i realized my mistake. he's very good so i kept him.
on D, i still run a base 3-4. i don't tend to pay attention to this scheme as much. i like getting guys who are 3-4 but it isn't easy or as necessary. for instance, i like to move 43DTs (290lbs and above only) to 34DE. for a long time, i couldn't find MLBs who were 34 tacklers so i used a couple of 43 prototype guys, however, i've just found two MLBs who are 34 tacklers. my OLBs need to be pass rushers but i currently am having to draft 43DEs and move then to 34 OLB because of the dearth of 34 OLBs. i've moved some gems but i stick to a rule of them being less than 270 lbs to be realistic.
as for the secondary, i prefer prototype or balanced CBs. same goes for my safeties. i've found that if you draft a non prototype or balanced defensive back and edit his position, when you return him to his original position, he often becomes a prototype or balanced guy.