I chatted with Corbin
There is a bracket to support the seat pan and that will need to be taken off to put the solo seat back on, but they assure me it isn't difficult and would be quick. Less than 10 minutes to switch seats. That's pretty close to ideal.
So, at this point, I'm thinking of saving my pennies for the Corbin with a large oval back rest. When 2-up my passenger will have a nice ride. If I want to go solo with my original seat, it's just a 10 minute change.
After all this... I'll probably end up liking the Corbin and not bothering to switch back. haha. I can easily see myself doing that. But it's nice knowing I'd have the option. I'm all about options!
I hope this exercise helps someone else asking the same questions. When I get the seat (I think I've passed 'if' at this point) I'll post up my impressions.