Life estate-is conveyance revocable?

A mother transfers conveyance of property to her son (no living spouse). She can live in the house til she dies, but the "deed" is transferred by her, the guarantor to her son. Can she change her mind at a later date, and transfer the deed to her daughter?
Revocable or irrevocable transfer? Can the son take out a home equity loan on the paid up house since he is the actual owner?

I am a law school graduate. What I offer is mere information, not to be construed as forming an attorney client relationship.
Has she transferred it yet or not?

She can do this if she hasn't transferred it: life estate to son, then to daughter.
to son for x amount of years, then to daughter.

If she already has transferred: is son living in house too? Does he have physical title? If so, then no luck. If not, then your gift could be considered a future gift with no present intent to transfer. If such is the case, then you can revoke and give to daughter.

Hope this helps.