Server Upgrade

I am building a new server to take over for an old one. I need to set it up exactly like the old one. However, I must have both running at the same time while I test the new one, transfer files, etc. Once the new server is established, I’ll pull theplug on the old and bring in the new. Old one is ‘FNSERVER’, domain is ‘FRIGIDNORTH’. We have 22 stations on the domain.
How do I configure the new server to access the old one, be able to log on to the new from a station (for testing). And still be able to change the server name and domain name back to the original’s once the old server is unplugged? Is this possible? Can’t seem to get the hang of these trusts.
Thanks