MIL has Dementia and is becoming defiant about a long discussed plan to move to a personal care home of her choice. How do we handle this?

You won't get her "on board" anymore... she has dementia. Husband needs to make decisions in her best interests, and it seems like it is beyond a rational discussion so your DH needs to just put in motion the decisions that were "long discussed" and the facility will help him make a plan on how to transition her with the least amount of drama.

I don't really understand what you mean by a "personal care home". Clearly, with dementia and an inability to understand her deficits and her need for care, your husband/her son needs to act in her behalf. She is very unlikely to either accept this or to agree to it, but it must be done despite that. Her defiance may last, and it may not. The home may suggest you not visit for a while to assist with her adjustment time. I wish you the best. When someone has dementia things are no longer a matter of defiance or cooperation. They simply need to be done.

Drop the word defiant. Stop seeing this situation as adversarial.

Your DH's responsibility is to act for his mother as she loses her ability to make decisions for herself. In doing this, he should as far as practical make decisions which are in line with what she wanted when she was of sound mind.

You describe the plan as "long-discussed." What were MIL's views during these discussions?

What are the main objections she is raising now?