If the chain is measured for the 56 it will probably be OK for the smaller crankset. The issue may be chain take up riding small/small with the 38 but that's easy enough to find out.

Keeping chains matched to the cassette is more important than to the rings,so from a wear standpoint staying with the same chain makes sense. OTOH if you ride both combinations roughly equally, having two chains that you rotate will keep them both matched for wear. Rotating chains is SOP for many high mileage riders, so you'd be in good company doing so.