Sue,
It involves some cutting and fabrication, mostly to the rack. Then the rack is welded to the subframe.
You would remove the LS seat completely, if you were going to use the repro solo seat (VERY comfy!). So the handles are not an issue.
I plan to eventually buy a new dual seat. I bought an extra subframe from one of the folks here, but it is from a 1st gen, and I wanted a 2nd gen (minor differences). I didn't have the heart to tell the seller - it was my fault for not asking enough questions.
Anyway, with that set-up, I could switch back & forth in just a couple of hours!