Can i amend my 2018 return changing the American Opportunity credit to Lifetime Learning credit, and if so how? Do i have to fill out form 8863?

Yes, 2018 can still be amended. You will have to paper file it but that is not a problem. It will take a long time to process. You can go ahead and  mark that you have not claimed the AOTC for 4 years. It will all be processed and straightened out before the IRS gets to the audit process.

Here's how: 

• Save a copy of the original return before doing an amended return.
• See how to file an amended return.
• Pay additional tax. Taxpayers who will owe more tax should file Form 1040-X and pay the tax as soon as possible to avoid penalties and interest. They should consider using IRS Direct Pay to pay any tax directly from a checking or savings account for free.
• Track amended return status online. Taxpayers can track the status of their amended tax return in English and Spanish using Where’s My Amended Return?  Amended returns take up to 16 weeks to process and up to three weeks from the date of mailing to show up in the system.