A friend did this successfully with her mother, but did not do it the traditional way. She and her mother bought a house between them and divided it into two self contained flats, with separate services and separate ownership, so that at any time one could sell their flat independently of the other.
When her mother died, my friend was able to sell her flat independently, which meant that her brother got his share of the inheritance and the same could have been done had her mother gone into care. My friend then stayed on in her own flat.
I think wtzend makes a good point over paying for care. We talk about elderly parents left homeless if children have to sell or relocate, but a family could be forced to move because an elderly parents capital has to be liquidated to pay care home fees.