We faced the same issue in 2006, when we switched software. At that time, we were paying a hefty fee -- and the only way to keep those old posts going was to continue paying. Or, have someone do the extremely laborious task of reconstructing the entire site from text files. (Those files are still available, if anyone is interested!)

I think we will be able to keep the current system going in read-only mode for quite some time. Could be wrong, but I think simply disabling new posts would do the trick.