If you're sticking to the same sort of connection ie, not upgrading to fibre then the old router should work. If they do send you a replacement router it might be "better" than your old one (more powerful etc) but anything you have connected wirelessly will need to be connected to the new router if you decide to install it. I would try using the old router, only installing a replacement if you have problems, as it will save you having to manually reconnect any laptops etc.