Lights would be no problem, if the connectors are not the same, just cut them off and splice into the harness. If you look around I think you will find a lot of hybrids. Exriders.com usually has several on the forum. There used to be a lot of people who ran 400 or 250r hybrids with either yzf or crf motors.
As far as switching, doesn't sound like a bad idea to me, I always like the comfort and ride of my 400 better than I do my 450, on most days I do.