Welcome to FT! Hope you have good experiences here, and on Frontier.

I have a very similar situation...I'm Summit, and while my corporate travel network recognizes my status and lets me book exit row, it won't let me choose stretch. So far I have *not* found myself automatically upgraded like you have. Three things:

(a) You can (of course) always call and request a seat change. If you use the Summit res number you shouldn't have much wait, if any.

(b) The new website will hopefully allow you to change seats on an existing reservation. Old Midwest Airlines flyers could do this, and many of us really miss this ability since moving over to Frontier.

(c) You can change seats at check-in for free as Stretch, and while it's always possible Stretch will be full, it's rather unlikely there would be no Stretch seats available if you check in ahead of time on the web. This is what I usually do, but many of my flights are on Ejets.

Currently, the only people who can pre-reserve Stretch are those who buy the Classic Plus fare (not very many) and Summit flyers who book directly with Frontier or who book elsewhere but then call Frontier to be reassigned. So at 24:01 before departure, it's very likely that many Stretch seats are open. Most likely you'll still find open Strech seats when you check in as long as you don't wait until the very last minute.

I don't know if/when we'll be able to choose Stretch via indirect booking tools like a corporate travel department or system. But hoepfully the website upgrade will allow us to change seats. In the mean time, you're very likely to be able to get Stretch when you check in ahead of time, or if you don't want to chance it you can call.