if you want to preserve the clock memory then use an ungrounded soldering iron. Usualy the cheaper ones are ungrounded and the midlevel ones are grounded. expensive or micro stations will sometimes have a ground lift switch. if you use a grounded one it'll run a chance of getting into a fight with your bench supply over weather 3v gets to be at the node youre soldering.

Good luck. if you find that the through holes are too small to get good heat transfer, don't be afraid to leave a 3/8" or so bit of the old leads sticking out and solder your replacement onto them. covered with heat shrink they'll be fine.

they make low acid or non corrosive or whatever hot glue to attach that little toilet lookin' doohickey for just these applications.