(A) It is possible to remove the model's ID from the sub-account, and starting anew with a new username as an independent broadcaster. The studio owner can contact support through their registered email, with the inquiry to remove the model's documents from any studio accounts it may have been linked with. The studio will keep ownership of the account usernames, and will remain free to reassign them to other model(s), while the model may use their documents to verify a new independent broadcaster account.
(B) It is also possible to preserve the model's account username while unlinking it from the studio, and making it independent. Similarly, the studio owner can contact support through their registered email, with the inquiry to unlink the sub-account from the studio, for it to become an independent broadcaster account. While they won't likely read your contract, it helps to make clear that it is per mutual agreement, with no impeding contractual obligations. Support may ask for verification through photo proof of documents from the studio owner and/or the model. This may vary from case to case – I don't know – so please just comply with whatever requirements are stated in the follow up e-mail to your support inquiry.
(I just went through B–by the way.)
To minimize the chance of triggering any alerts regarding document forgery and such (I don't know if these can happen; I only read about it on this forum), it seems wise to refrain from attempting to verify new usernames, until the studio owner and/or model have complied with any requirements from support towards either removing documents from the studio (A), or changing the account's ownership and status (B), and after verifying that these changes have become effective.
