Trimmed surfaces are sometimes tricky because, once they are trimmed there are certain commands you can't use on the trimmed edges. Useful commands that allow to create or maintain continuity, such as _MatchSrf or _Symmetry become useless.
A possible workaround to this issue is, if appropriate to the situation, to _Untrim the trimmed edge and then to _Split the surface using the command line Isocurve option. This option allows you to keep the same geometry structure for your surface. If you turn ctrl pts on (F10), you'll see all points lie on the surface edge. You can erase the unwanted split part of the surface and use any command you want on the resulted surface!