There really isn't anything like an asset entity, that you can just change the attributes of.
All assets are defined inside trust lines, and each holder has its own definition, so to say.
What I would do is to create a new asset, "BAR", and then put up an offer on the DEX, selling "BAR" for "FOO", at a price ratio of 1:1.
Your holders will still have to open a new trust line to "BAR", and remove the old one to "FOO"
