Unless your extension request was rejected, you won't need to revise the 2017 AGI.
If your extension request was rejected, please see the third paragraph of the TurboTax FAQ linked below. You can re-submit a rejected extension request until April 20, 2019.
https://ttlc.intuit.com/questions/1900551-can-irs-extensions-be-e-filed
*** 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.***