Switching to Qing gives free conversion to Confucianism. I can flip, declare on Ming with mandate CB, start converting a Coptic province, accept demands the next month and be Coptic again before the war is over.
Another random buggy mechanic that affects whether to take mandate is that flipping to Manchu or any other tag doesn't downgrade government rank as HRE emperor, but flipping to Qing does. The only way to fix it is to become EoC or Tsardom which are locked to empire rank. The state limit penalty for Duchy doesn't matter but the -1 leader without keep and diplomat hurts a little.