I dont think it will do them any damage if you leave them in. You will just have a lot more trebble as the coxials have inbuilt tweeters. I wouldnt recomend upgrading them as the point in coxials is that its an all in1 speaker. Do you have a sub or just stock/6x9s in the back?
are the oem tweeters in dash connected to the hu or front speakers?
yer sub and amp be in the boot!
So there would be too much treble if the Coax were in rear?