Tunghaichuan, so what is the best approach?

New lead free solder requires more heat. Heat can destroy components.

Wouldn't a 200 watt soldering gun or more powerful than 40 watt iron heat up the joint faster at the local site and penetrate less net heat into the component; thereby heating the joint site to required temp fast, do the soldering, and get out of there?

Use leaded solder over the old new solder joint, clip old wires, and use low power 20 - 40 watt pencil iron?