There is no charge to amend a return, unless the change you are making requires an upgrade to a higher version of the software. If that is the case, then you are charged for the difference between the amount you paid already and the higher amount for the upgrade.
Wait until your original return has been fully processed before you amend, and make sure you have saved a pdf of the original return before you amend.
See this TurboTax support FAQ for amending a tax return - https://ttlc.intuit.com/turbotax-support/en-us/help-article/tax-return/amend-change-correct-return-a...
Do not expect quick results from amending. It can take four months or more for the IRS to process an amended return.
You can watch for information here:
https://www.irs.gov/Filing/Individuals/Amended-Returns-(Form-1040-X)/Wheres-My-Amended-Return-1
**Disclaimer: Every effort has been made to offer the most correct information possible. The poster disclaims any legal responsibility for the accuracy of the information that is contained in this post.**