Yes, you can have an anterior approach ptosis repair after a posterior approach ptosis repair. There will be scarring in the plane of the levator muscle but an experienced oculoplastic surgeon should be able to manage this. It you have any excess skin and/or fat in the upper eyelid, or need to have a lid crease formed, a primary anterior approach may be a better option. Revision surgery is always more difficult than primary surgery and there is usually a long period of time with swelling post-operatively. Good luck!
Repeat eyelid ptosis surgery, whether after posterior or anterior approach, is more difficult than original surgery so best to get it right the first time. Having said that, revision ptosis surgery is possible. See following link.
External ptosis repair after prior Muellerectomy is in fact possible, though slightly more challenging . The real question is weather Muellerectomy is the right choice for you as the primary technique. If you have mild or moderate ptosis, then it may be a good option for you. An experienced surgeon may be able to may be able to improve a mild asymmetry. And despite some assertions, upper sulcus deformity and platform asymmetry improvement is also possible. You should choose a surgeon that is experienced in both techniques and have him perform a detailed examination so that he can choose the right option for you.On my website you will find examples of both external and internal ptosis repair in the gallery section.
Anterior ptosis repair, meaning a skin incision is made and the levator muscle is adjusted, is probably the most common way we correct ptosis. It is arguably more anatomically correct, because you are repositioning the tissues in a way that they function better. You can also remove excess skin when necessary, form a proper lid crease, and rotate the lashes upward. Posterior ptosis repair is good in certain circumstances, such as when you have a mild amount of ptosis (2mm or less), you have no excess skin, and you have a symmetric lid crease. This is mostly done in children and younger adults. See an oculoplastic surgeon who can go through all your options and can perform the best procedure for you.
Revising the eyelid after the mullerectomy is highly challenging. Not impossible but this is the best opportunity to get things done correctly. Posterior mullectomy will not correct your hollow sulcus, the eyelid platform skin, or the eye lash ptosis. This is your face. You will have to decide for yourself if getting the service for free is more important than getting the service right.