Is there a way to modify an existing tax year that I've started with a modified return of a previous year (one different than the one I referenced when I started the existing return? For example if I need to revise last year's taxes, can I re-reference that prior year return in my present year return that I've already started? Or do I have to start over with a new current year return?
No. You can only transfer 1 time before you start 2018. If you want to transfer from the Amended return you would have to clear your return and start over or start a new return if you are using the Desktop program.
if you are using the Online version and want to clear and start over to transfer I can tell you how.
Otherwise you will have to enter the changes into your return manually.