Is there anyway to improve or reverse rusting shells? Waterfowling in rain and don't really want to toss out $40 boxes of shells.

I've used 10 year old slightly rusty shells befor . I took some fine sand paper to a few but did not have any problems when using them . All were fired in a 30 year old 870 . Like you stated they are too expensive to throw away . I'm talking a little rust on the surface nothing on the seal or primer .

Happens quite a bit. Take some Rem Oil /WD40or any oil and wipe them down while using a bit of steel wool to clean them up. If they aren't pitted horribly they should cycle. If the rust is corroded or made it around the primer, I would discard.
Without pictures it is a bit hard to give perfect advice but, I hunt ducks and birds this time of year a couple times a week and have had to clean shells I've left in the bed of my pickup and typically it's no issue.

Double

Car paste wax.

I would throw then in a vibratory tumbler with corn cob media and some polish.