Does upgrading mid-month charge me twice?

No. It prorates. You get credit for the part of the month you haven't used and pay the difference. The modal shows the exact figure before you confirm, and that number is what the card is charged — it's computed server-side, not estimated in the page.

What you don't get is a refund of the used part, which is the normal way round.

Came out at about $27. That matches what I'd expect. Thanks.