I will be amending my 2018 return. The 2019 turbotax program is still using the original return. How do I tell the program to switch to the amended file?

"I will be amending my 2018 return."

If you have not yet amended 2018 how would you expect the 2019 program to switch to an as-yet non-existent amended return?  If you want to amend 2018 and transfer the amended 2018 to your 2019 return, then you have to amend 2018 first.  A 2018 return can only be transferred to the 2019 return before you begin to enter any other data for 2019.  If you start entering 2019 data, it is too late to transfer 2018.