The fact that you filed on May 17, 2019 does not give you three years from that date to file an amended 2018 return. You must have filed for an extension to file your 2018 return. If you did so, your plan should work. After filing the amendment and paying the tax due, you can expect a bill from the IRS for late payment penalties and interest. That will wipe out about half of your $900 savings. https://pocketsense.com/interest-penalties-amended-tax-return-19397.html
Instead of just unclaiming the AOC, you could claim the Lifetime Learning Credit, for 2018, $1600 x 20% = $320.
Here's a post from another user who claims you can't do that, but does not cite a reference. https://ttlc.intuit.com/community/after-you-file/discussion/can-i-amend-a-prior-year-to-unclaim-the-...
