Believe me, there is no part of the stock stereo in a 91 Accord that you want to keep, including the front speakers. Replace them with some other 6.5s, especially if you already have them! There might be some depth issues in the doors, but nothing some MDF and some cutting can't take care of.

The stock speakers are connected with a wiring clip, which needs to be cut off, and there is zero extra length in the wire, so you're going to have to either attach some new wire or, if you're running an amp, just run all new wire.

Dan