My wife and I didn't have our 2017 AGI handy when we just filed our Federal extension - we later found it, which was different than the AGI we filed (we gave an AGI based on an older document, which ended up being slightly different than the actual number).
Is there any way to amend the extension (does it impact the penalties on taxes owed)? The software doesn't appear to give me an option to change it.
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.