Strategies for dealing with an old RT system

I set up a test RT system a while back as we did planning to build our
production service. As to be expected, despite my warnings, several
groups started treating the test system as a production service, and
have an awful lot of knowledge stored in the system.

It doesn’t appear there really is a way we can import the old info so it
looks like I’ll have to keep the “test” system alive for a couple of
years and just have a cutover date where we ask that nothing new go into
the old system. Luckily the “test” system used email addresses like so I don’t have to worry about replies to
old tickets ending up in the new system.

Any suggestions? Has anyone dealt with this before? Could there be a
neat solution I’m unaware of?