Retopologise part of a mesh?

Hey guys, I was just wondering if it is possible to severely retopologise part of a mesh (ie remove entire faces, and extrude and rebuikd a section) that has multiple subdivision layers? I have tried altering the base mesh and reimporting but because the mesh is altered and not just retopologised, the projection of the higher levels spazzes out. I have a finger on a hand that has somehow resulted in totally bizarre edgeflow. Vertsare overlapping, parts of the mesh are folded in on themselves, its an absolute messs and needs to be redone. How can I do this without retopologising the entire character mesh? All my problems with the methods ive found online come down tot he fact I have multiple subdivision levels.

Heressome pics:

Please cna soemone help asap.