Can you not edit your whole league's information now bc of this new GM mode??! Please tell me that's not true! Could this be because I haven't been able to get my key loaded and am still in Demo mode? If not, this game has become about 75% less interesting to me...

Do I have to do this in Sandbox? Is there no way to circumvent this so I can at least maintain records from year to year?

Career/Sandbox/Online have all remained mostly the same between versions. I don't think we changed anything for GM mode as far as what you could edit.