Most promotional discounts apply at the time your subscription began. Since you've used a trial version of QuickBooks Online (QBO), I highly suggest contacting our Support team. They have tools that can pull up your account in a secure environment and check if you're still qualified for the promotional price.
Here's how to reach them:
In QBO, go to the Help (question mark) icon at the top right.
Enter your concern, then select Let's talk.
Tick Contact Us to connect with a live support agent.
Choose a way to connect with us
To learn more about how to subscribe to QBO and as well as understanding the rates, consider checking out these articles for future reference:
By default, the discounted price is not applicable when upgrading/downgrading your plan. You will pay the trial license and upgrade it with regular rate. I am not sure but you may contact Support to try your luck.
You may pay your trial account of QBO Essential with regular rate and open a new blank QBO Plus account with the current discounted rate.