You can modify your snap store username (the id) by navigating to OpenID transaction in progress, amending the field and clicking save.
This is different to your display name which has a known issue with changing back as builds of your snap(s) complete. See the link below for my long sordid story there - I’m purposely not updating my macaroons because I want the store to get fixed to allow pre-existing macaroons to not update the display name unless the macaroon has newer data than the store because you changed your login data after you last used the store and then created the macaroon with that newer data - By my not updating the macaroons I’m using I’m being a guinea pig for the store team to use to get it right (plus I have LOADS of snaps so changing them all manually is a long and arduous job, and I just don’t have the energy to do it):