Regroup

No. If he is a business trainer, she gets ONE extra business star on the regroup plus a small slice of whatever rank & nobility he's got.

If you are hunting for items to buy from various stores in campaign, I would hang on to Arnold a little bit longer if I were you (unless you really need the space for someone else). If you are done with him in that respect, only then regroup him.

Just make sure you do the regroup the right way round - select Sophia first in the HQ, THEN regroup, THEN Arnold. And remember first to clear out any items he is carrying, or you will lose them as well.

If you have other generals with higher trade stars than Sophia, it's possibly worth regrouping him into them instead, as you get closer to the magic five earlier.