That option is already long, long gone. Think they quit selling those keys about a year ago. Although, they did just give a bunch away for free in the last week or so during twitch streams.
Where are you still seeing the ability to upgrade to the Elite Founders?
Edit (added below):
My bad, just noticed you were posting from a different game forum. I was browsing the GalCiv3 forums and spotted this thread, and my post is totally in regards to GC3. No idea on the answer you are looking for.