According to the TurboTax FAQ below, the TT program will allow you to allocate the lump-sum payment to both 2018 and 2019. This will help on your tax situation, of course, but you'll also see that there is no need to amend your 2018 return.
I believe the info in the FAQ will tell you all you need to know. But if you still have further questions, please post back.
"What is a lump-sum Social Security payment?"
https://ttlc.intuit.com/replies/4767454
*** I am NOT a tax expert. I am a seasoned TurboTax user, and volunteer to provide assistance to TT users. Nothing I post is to be considered TAX ADVICE; I bear no legal liability for responses.***
