Hello,
You may receive an offer to buy up to business, similar to AC's last minute upgrades, at check-in.
You can purchase an economy plus seat anytime by calling in, accessing your reservation at United.com and when checking in(if space is available).
Free E+ isn't a published benefit for non United premiers but they may give it to you if you are *G/*S by asking nicely at the gate/club/check in if space is available.