How do I replace speedo cable housing

Hi, I could really use some help with this.

So a while back, my speedometer cable broke. When I went to replace it, it came off fine, and I found where the cable had broken, confirming the cable was in fact the problem. But when I tried putting the new cable on, it wouldn't thread on right to the housing on the transmission. I noticed the threads on the cable housing looked to be in bad shape. I need to remove the cable housing to try and fix the threads, or replace the housing altogether. When I went to remove it, trans fluid gushed out, so I quickly put it back in. So my question is, am I going to have to drain out all the fluid in the transmission to remove the housing? I'd really rather not, since the fluid is nearly new.
Author: 66FuryBoy