Restoring ISE 2.7 backup onto 3.1 should work with no problems. If you keep "ade-os" option off the restore won't include the hostnames nor the IP addresses details. One thing to remember is that ISE 2.7 licenses need to be converted to ISE 3.x model prior to the upgrade.
To do that you should engage with Cisco licensing team providing them with the SO# of your 2.7 deployment, company name, and how many nodes you have, when you are planning to migrate to ISE 3.1, and then they will convert the licenses from 2.7 > 3.x decommissioning the old licenses. However, they would also create a new temporary licenses set for your ISE 2.7 deployment to allow you to revert back to the old environment in case it is needed, they might ask you how many licenses should be generated for the temporary set. You should find all the temporary and the newly converted licenses in your smart account after the licensing team finished working on your case. You should also migrate TACACS licenses if you don't have them already in your smart account.
Once you are happy with the newly created deployment you can inform the licensing team that you don't need the temporary licenses any more and they will remove them from your smart account.
Also, please remember that if you will keep the new IP addresses on the new PSNs, you should change the RADIUS/TACACS servers details on all network devices to reflect the new IP addresses.
