You could remove the part of your save, via save to file (in a text editor such as Notepad / Notepad++, as the .coc2 file can be opened and edited as text) where it says: "wyldMark":num,
After that you can save the changes you made to the .coc2 file, and then apply a new Wyld Mark via the Wyld Wine TF item (also, you can apply a reagent to the Wyld Wine to get a specific Wyld Mark).
Please do this with a copy of your save though, in case anything goes wrong (save to file, and then a copy of that file).
Bear in mind this will make it so that your save counts as save edited.
Also worth noting, the removing of the Wyld Mark via the above won't remove the TF that the previous Wyld Mark applied, so if you didn't want your mage to have Cloven / Goat legs anymore, you'd have to change that with some other TF item too. Removing the Wyld Mark from the save only allows you to get a new Wyld Mark, it doesn't remove the specific TF of the previous one - which is notable for the Equine Anus Wyld Mark, as currently there doesn't seem to be any method to get rid of the Equine Anus besides Save Editing, as no TF items seem to remove it.