You don't need to if you switch to standard GEDCOM structures instead of using custom tags.
The standard GEDCOM way is to have multiple names per individual. Each name can have a type:
I think the following mapping might work for you:
* farm -> estate
* formal -> birth? (question is what "formal" really means other than "not informal" which is for instance a nick name)
* formerly -> change (personally I find it difficult to handle name changes without making clear in notes which name was used in what period)
You can tweak descriptions with custom translations which do not get lost with updates.
Another solution is to clone the git repo, make a branch of your own to do the tweak in, and pull changes from master at every update.
I agree with Bert 100%. GEDCOM has a wonderful, open and totally transportable set of generic tags already built into its structure.
For multiple names just add a second name tag and use the TYPE tag to identify the type of name added. I do this regularly in my database. This is far better than creating new and generally hard to support custom tags.