If it's not "technically possible" to change the guild names, that heavily suggests that the guild name is used by the system as a unique identifier for the database table record for a guild. While that's a pretty big design no-no, there are obvious workarounds. One extremely simple one I can come up with off the top of my head: create a new field for the actual guild label, set the value of the new field to the value of the id field, then build a simple UI to allow Guild Leaders to change the guild name. No need to re-map or transform any data. Now you have one field with the unique identifier, and one with the guild name.
If they say it's not technically possible, that's a red flag that someone goofed BIG TIME when they wrote the code for guild creation, and now it's too ingrained in the core of the app to fix without a major overhaul and a lot of Dev $$$