Lure her. Put her in a sit and slide a treat under her nose so she has to collapse down to get it. You may need to use a new word for Down such as Platz. If you trained her to roll by saying Down, Roll. Then she's associated the two words as the command to Roll. I screwed up Come that way. Come, Heel...she automatically does a Heel now.
I am going down the same street right now you are. I would always have him sit but then I would go tell him down. Now when I say sit he lays down lol. He thinks its the same command. It was my fault he is doing this so I cant be mad so we are working on sit all over again. He understands sit but knows I am going to say down next so he just goes down so he can get his treat. Silly dogs
Then correct for that. I would withhold the reward if he gets it wrong. Throw another command in there to mix it up for him.
Sit, Stand, Heel. And teach him to Down from a stand...same way as when you taught from a sit with a lure so their front feet go down first and their body collapses
He knows to sit I just think he likes to skip the sit when he knows he is going down right after. While healing he knows what sit is. Its just when him and I are messing around he does it. If I am being serious with him he knows to sit when I say it. I just think its funny he knows when I am being serious and being silly with him because he is silly with me right back.