I don't think anyone is seriously asking to have GP suddenly be maxed when they change to their DoL classes. Rather, the complaint seems to be that GP stops regenerating at 400 while you're not in a DoL class.

I don't see why DoW/M and DoH classes couldn't just have 1,000 base GP, rather than 400. This would allow even the best geared DoL players to use their DoW/M and DoH classes to do things while waiting for GP to build back up, and would avoid accidentally resetting GP to 400 when one needs to quickly change to a non-DoL class. 1,000 GP would be more than high enough for at least another expansion or three.

It would change absolutely nothing with the rate of HQ obtainment, as GP would still only regenerate at the set rate.