Doing a cold re-arch is a temporary fix at best. Once a spring has sagged, it will return to that shape pretty quickly. The proper way to re-arch a spring leaf is to anneal it, re-establish the correct arch and then retemper it. Very few spring shops and virtually no hobbyists have the equipment or the metallurgy experience to do that.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!