Usually, you don't even want to remove the PoS router you get from your ISP from it's box. In my experience no ISP ever gives away decent equipment, it's just the cheapest thing they could order in quantity.
Another issue with an ISP-issued router is that you have no idea what they did and can do with it. I know of several ISP's that have full remote access to the routers they hand out. Basically, you risk exposing your entire home network.