Well a couple hours into this I think I figured it out… if you skip columns then it simply deletes the data that was there… and boy that is unexpected and problematic… Luckily I had the original export to re-import…
in the end I used the exported csv and modified what I needed and that seemed to work
I also used google sheets -> download as .csv given some peculiar issues that may or may not be a result of Excel on macOS.
I’ll also note I’m on a pretty old version so perhaps this process has improved with time?