I would guess you could upgrade a lifetime single to a lifetime with spouse/partner for the delta cost at the time.

I know when I upgraded to Lifetime about 10 years ago, I got credit for the remaining dollar value left on my 3 year membership. It was about $2K at that time and CO did give notice that an increase would be coming, so you had a window of time to get in at the older price.

Note: I waited until after the bankruptcy issues with CO cleared up before I bought the lifetime.