Unless you have a bunch of other .325 saws that can swap bars and chains with the 400, I'd just leave it at 3/8LP. The 400 will pull better with that anyways. Keeping one more file size around is no big deal.
If you spin your own loops, maybe. It's a performance hit for that little saw in exchange for some perceived convenience. Assuming your 325's are 050 it should be as simple as a rim/spur swap. Then again, a lot of Echo bars are 1-2 links off from Stihl and Husky chains.
Looked into this recently, trying to match up to a pair of 16/18" Stihl's. Would have been pretty easy to match up to husqvarna lengths, but stihl was consistently off by a couple of lengths in ever combination I could come up with.