1) Will it be a problem if we've already updated TurboTax to the newest software revision?
Updates usually don't effect the return ... but if it did you would have to put the return back to how it was originally.
2) If so, (short of retyping the original numbers) what is the best & easiest way to amend the return? For example can we somehow re-install the "old" TT?
Since you messed with the return since you filed it did you by any chance save the original unadulterated .tax2016 file ? When you use the downloaded version you have the ability to save multiple files so pray that is what you did.
3) If not, do we amend the ORIGINAL or the changed returns?
If you did save the original return then you will amend only that one and when you are done make sure to save the file under a unique file name so you know which one it is.
