For individual models
They must contact the studio to add them to their roster. This will be blocked as a duplicate account, so the studio should then contact support to unblock this and block the original account.
NB. Once you are attached to a studio, only that studio can detach your account. Support cannot do that upon any model request.
For studio models
Some studios will have a block on any model working for another studio, so first of all, the model must contact support to see if this is something they need to be aware of.
If you want to change studios, the studio you work for must first release you. You can then have the new studio add you to their roster of models. If there is any issue with this, please contact support.