If your girth outgrows a cylinder, you can still use it to work on length and glans expansion. If you want a mushroom head, just use your old 2" cylinder instead of buying an expensive specialized head cylinder. Since the sides will be packed, the vacuum will focus on the glans.
Some guys use up to three cylinder sizes. They start in the smallest cylinder for the first set, the middle size for the second set, and the largest one in the third set. This allows you to progressively pack each cylinder.
Otherwise if you really want to get rid of you old cylinders, you can always try selling them on eBay, or one of the Penis Enlargement forums that allow it.