There haven't been many DP CA helmet threads, but of course, the repositioning could have happened in another thread, making it harder to find. The Don Post helmets in general, as well as the Rubies helmets, have the same issue as the DP CA and possibly a Rubies Supreme dome repositioning thread could help out?
There are two ways - either trim the T-mount at an angle from front to back or cut around the arms of the T and halfway down the base of the T down at the crown surface of the mask, then heat the plastic and push down the T-mount and hold it until it cools, then put on the dome and work it until the right position is reached, and then glue the T-mount in place.
I found this Rubies Supreme thread - may be useable as inspiration until the DP CA thread pops up.
dome-mount-cut-or-not-vt3627.html