Sorry for the delay, still getting over all this crud.
Do you have a save that you can "test" this before doing it? I have been reading all the threads for this bug and what crin has posted also. Let me know if you can test it in another save and the results.
Shannon
No worries! I made a new save and tested it out, and it does work. I took some screenshots too so you can see. Here:
These two show where the companions option is not open, but in the relationships panel you can see their bar is full. The dogs name is Yuka. The weird blue square on the lower right is because I tried to take a screenshot but somehow only took a screenshot of like one little square of the screen, and my screenshots always hover in the lower right corner for a minute before they disappear.
This one shows Yuka up for adoption.
This one shows the adopt option showing up for Akira, so you can see he's the one who adopted her this time.
This one shows that their relationship reset.
This one shows the companions option showing up after rebuilding the relationship.
This one shows that they successfully became companions.
And this one shows that the milestone successfully checks off now.
EDIT: Sorry, I forgot to mention, re-adopting Yuka did reset the dog's traits as well as the relationship bar. I'm not sure if that's going to be relevant to your decision or not, but I thought I should mention it. Let me know if I can move forward with this in my real file! Thanks.