Some shops have the policy of refusing to fill scuba cylinders that were made before the 90s. They allege that they are made of a "bad" alloy. The truth is not as simple as that. Aluminum alloy 6351
occasionally develops sustained load cracking in scuba tanks. There are well documented procedures to test these particular tanks and catch any developing failure before it becomes an issue.
Luxfer discontinued the use of 6351 alloy in June 1988. If your tank in question was made by luxfer after June 1988 (see original tank stampings), then it is not made with the "bad" alloy. If your tank was made before, you can call luxfer, read them the serial number stamped on the tank and then they can confirm if it is made with 6351 alloy or not. If you do indeed have a 6351 alloy tank, have it annually eddy current tested in accordance to luxfer recommendations. You can safely keep on using your tank until it fails to pass an annual test. Click here for more info:
Sustained-load cracking (SLC) in ruptured scuba cylinder made from 6351 aluminum alloy - Luxfer: Setting The Standard Worldwide
Are you sure that the shop that told you about the "bad alloy" did proper testing before condemning it? It's sort of convenient for them to say that you have junk and should buy a new tank from them...