The presence of this assignment has a code smell.  I'm thinking maybe we should just have saveAccountDef take an accountId directly and just grab the accountDef off of that?

And maybe instead of copying the defaultPriority onto the account, we should just have it be a getter that pulls it out of the accountDef?

And in terms of this comment, I don't see us implementing a specific bulk item reordering API for accounts, and in the event of making a single account the default, given the sort order you have established, all we would need to do is splice the account out from its current location and insert it at the 0th location.