This will be little to no help, but maybe you can get SOMETHING out of it.
I'm drawing a blank on Unity 3.x. In terms of licensing, if Unity 3.x still used the dongle, you will need it on the temp server. I don't think Unity services will start, but maybe you don't need them to.
You will have to have TAC convert the dongle to the FLEXlm licensing scheme for your 4.2 server. So you need that TAC case anyways.
Since you are using new hardware anyways, build your temp server, so a DiRT backup and restore, see what happens without the license key. Not the answer you are looking for, but it might be worth practicing the upgrade anyways. I'm not sure if passwords or some other things would carry over. These are things you can document in your implementation plan so you don't run into surprises during the real conversion.