Legal Question in Wills and Trusts in California

None. Living people don't have any heirs. That is a huge lay misconception. When I was in law school my professor gave an incomplete to anyone who was able to name their heirs.

Trusts are usually revocable during the lifetime of the settlor, and the settlor can change his or her mind and amend the trust up to his or her death.

Mr. Roach is correct with the proviso that if one Trustor has died, part of the trust may be irrevocable.

The previous answers are correct, but only for the most common form of trust - an inter vivos revocable trust. There are many other trust arrangements that, although less common, would give certain rights to the successor beneficiaries (the proper term for what you mistakenly call heirs) that can be enforced during the trustor's lifetime.