MikeMpls - I'm in your same shoes right now. I want to get a Lifetime membership once I hit Plat later on this year, but I also wanted to receive the sign up bonus miles for the 1 or 3 year membership.

I contacted the WC 800# (twice) to ask whether I get full credit for the 1 or 3 year membership I purchase if I later upgrade to a Lifetime membership. I was told that I would receive full credit only if I upgrade to the Lifetime within six months of purchase. If I chose to upgrade after the six months, I would be credited a prorated amount towards the lifetime membership.

So, I got a 1 year membership, got 2.5K miles, working on becoming a Plat, then upping to a Lifetime (all w/in 6 months...)