we'd have to run the two systems in parallel for the forseeable future

Yep. We wouldn't want to put this system in to 'archive' status (accept no new content) until the new system has been fully checked out and passed user acceptance testing, at the absolute earliest. Then we can push everybody over there. It may be that there are some users who simply refuse to use the replacement system. Not sure what to do about them. Leave the current system running forever?