remnants of old mesh is still showing

Hi.
The the blend file you import in the package must have the same number of meshgroups as the one you replace. That aside, you edited a full-body outfit to be pants, so you need to clone pants and import your blend there. But before you do that, make sure your mesh is split the same way and its meshgroups have corresponding cut numbers sims4studio.com/thread/13120/cut-numbers-easy-explanation
As for those stray faces in the upper-body area, just delete them.