It is only possible if you saved the SHSH blobs (containing Apple’s signature) used in each firmware to be able to update or restore back to that specific firmware in future even if that firmware is no longer signed.
Would be nice to do, but as others have said it isn't possible without saved blobs.
I have a 5 on 10.3.3 myself and would love to downgrade it. I have a early 2014 backup and tweak backup waiting. I've had to make do with using it on a 8GB iPhone 4s, which is 8x less in capacity than my 64GB iPhone 5.