I put 6.5 Focal speakers in my door.
I had to modify it very little. Any professional stereo installer with half a brain will be able to do the same.
Putting a smaller speaker ( 5.25) is the easy way to do it.

All they will do is take out your old speaker and cut an adapter out of wood to fit the new one.
You will probably lose sound because the cone will be firing sound into your door panel.

I have to take off my door panel to fix a lock motor this weekend, when i do i'll take some pics and post them.