Don't know how many times I've bought an elixir to change a pet, finally happy that I've changed them into what I wanted. Only to find I've come online one day to an update in art and don't like it anymore.

Any chance at an option to keep the old art instead of automatically changing to the new one? o.o

That will probably never happen, it just wouldn't make sense to have two versions of a pet with one that is "inferior" art. However there is a solution if not an official one. In Firefox there is an add-on called My Image Here that lets you replace any image on the web with one from your computer. Link here. You could save the pet image you wanted to use then just use this program to swap it out. The downside is that it replaces every instance of that image so if you go view another pet of the same species and color it would look like the old version for you.

Another option to just affect your pets is to just a Custom Overlay, which can again be whatever image you want. There's several different CSS codes you can use so if you need help you can go ask on the Web Design forum :)