You probably need to run configwizard again on the client PC's that have been switched. To do this, from the client PC, BROWSE to the licenseadmin server PC to the TCP5_USER share, then into TCP-Pro and click on ConfigWizard.exe. You only need to get a sequence number so uncheck the "configuration profile" option. Going to the next screen, it SHOULD show your existing info in the name and comment fields. If so just go ahead and click "Finish". If not make sure you put something in both of those fields and hit "Finish".
If you get an error from configwizard you have one of the following problems:
1) The TCP5_USER share is not writable for your client PC. You must make it writeable.
2) There are no licenses left. You can run licenseadmin on the server PC to see if this is the case. If the PC name is changing because of the switch from domain to workgroup you'll probably have to delete the user entry in licenseadmin for whatever PC you are trying to re-license.
3) You have a broken licenseadmin server or a version incompatibility (you can't use versions of Pro <5.0.81 with new servers of vice-versa).