That's not really the way it works though. Contracts can be renegotiated, it happens pretty commonly, and if she ever decided to try to legally terminate the contract, Britney could legitimately argue that as her current record deal was signed while she was a conservatee she could not legally consent to the contract and it was signed by her father thus making it void.
It's also entirely up to RCA/Sony's discretion if they fulfil her contract. They may be okay with the fact that she has retired and are accepting of it and maybe will just release a greatest hits/rarities collection some day.