This is similar to repairing jewelry. There are so many different ways of producing jewelry that there is no general recommendation. I suggest you should talk to a goldsmith who can test your jewelry.
< Old jewelry could have many different galvanic covers.
< Old jewelry could be extremly thin and hollow.
< Old jewelry could be stuffed with putty.
< Old jewelry could have a very low alloy.
< There may be synthetic gemstones on the jewelry.
< Old jewelry could be too fragile for repairing it.
< Chains made of 333 gold are often not reusable.
You can often remove the gemstones by sawing them out of the jewelry.
If you want to test the reusability of gold, visit:
Jewelry made out of Teethgold
You can test the gold or silver of old jewelry in the same way!
If the old jewelry is made out of 585 or 750 yellow-gold, the chance of reusability is much higher than jewelry made out of 333 gold.