Changing a roaming profile to a local profile is easy.... as long as the underlying user accout does not change!!!. However you imply that you will be moving from a domain to a Workgroup environment. If that is the case, you would need local user accounts, and you are probably better off just starting from scratch. You would need to create the local user accounts and profiles and then forklifit in some sections of the roaming profiles such as favorites, outlook nickname files, etc... into the new local profiles. Copies of the old roaming profiles will still be on disk for you to plunder.
If the domain will still exist but just no good connectivity to the server where the roaming profiles woul have resided, then you would need to remove the roaming profile location from the users AD account on the server, and then on that users workstation, go into system properties, advanced tab, user profiles and use the "Change Profile type" button, to convert the stored profile to be a local one.