Now, boss man comes in and has a new office opening up. Go grab the r1 box out of the closet, you need to upgrade the code and reconfigure it. Cable it up to your PC with a serial port, open some some sort of terminal program so you can catch the boot and password recover it. Plug it's ethernet into your lan, as you're going to need to tftp down new config, and turn it on.
Why are you putting a router that you know needs to be reconfigured onto a production network? This could backfire regardless of IPv6, since you could have a similar issue if the router was performing DHCP from a locally configured pool. If someone did this and complained to me I would tell them they just learned a lesson and now know better then to go connecting equipment with an existing configuration to a production network without doing a full review first.