When dementia is in the picture, local reasoning fails. It will be hard to convince your loved one to make the change.
Get the new place fixed up with some of your furniture and just make the move.
Let your loved one know shortly before that you’ve arranged a lovely place to live and drive on over.
Script yourself telling your loved one you’ll try it for a month and see how you like it, how lucky you both are to live in this new home, etc. They will hopefully benefit from your positive mood.
Their forgetfulness may be such that they will be “in the moment” and come to love and accept the move.