stop writing that one and write the other
if later on you want to return to the first one, that’s fine (and you got 20k words to work with)
if not, that’s also fine
just write what you want to and have fun with it
I’m not a gr8 writer or a gr8 tip writer so sorry I chime in but I did this with my last NaNo. Before last year I mostly had positive experiences and always managed to finish with my original idea and even last year I thought I had that shit down but about 15k words into the story I just wasn’t feeling it and I kept eyeing this other idea I had.
I felt a little guilty and like I was giving up but I just switched mid-NaNo and then it turned out great and I had fun the whole time doing it. I would’ve regretted sticking with the first story and I’m glad I didn’t even though I still plan to finish it at one point.