When you do 'edit as new message', a new signature block is appended below the existing one, and any changes you make to the signature used apply to the new block.

I believe it would be more useful to detect the signature block in the existing message and treat that as the signature. When the new message first appears the Signature: drop-down could show 'existing', or something, and you could retain that signature, or if you changed, the existing message's signature block would be replaced (rather than the new, appended block being changed). If you wanted to get really fancy, you can detect whether the existing message's signature exactly matches one of Evo's configured signatures, and display that one in the drop-down if so.