Following the examples in the webextensions documentation, I used my personal email address as an extension id. I’ve realised that it propagates in numerous places, namely the sync storage area. I’m not too happy with this choice and would very much like to change it. I tried modifying it in the manifest.json but I get a message warning me that the UUID doesn’t match with the id in my manifest.json.
Is there any way I can change the UUID? Would this have repercussions on user’s updates?
It is not possible unless you submit a new extension.
it is possible, with a cool hack!
Legacy addons can install other extensions and can remove themselves. You might say that you cannot create and sign legacy addons, right? Well not from scratch. But you can “upgrade” a webextension to a legacy one.
So… submit a new webextension with a new uuid. Then submit an update of your webextension with your personal email but as a legacy extension. Make the legacy extension do nothing except install your new webext with new uuid and then make the legacy ext remove itself.
No clue how you can migrate data. It will work only on versions smaller than 57.
Try this with unlisted extensions before pushing it to your users 
Good luck!