I can't speak to which you should get (most of my family always had the low tier, we don't need to go weekends), but WRT the upgrading, you are correct, you cannot upgrade after purchase, UNLESS/UNTIL the pass you want to upgrade to becomes available for regular sale again. When we renewed my niece's pass, this was explicitly stated to us (which I'll give them kudos on, b/c they didn't at the start of it all when we we did downgrade the one higher pass, and then couldn't bump it back up, we were kinda mad). I kind of recall they would have let us renew to anything since it was a now defunct pass (the Weekday select) but we still just opted for Pixie or whatever it's called so I didn't really pay attention/let her get through all that.