You cannot retroactively change how the property was held. What you can do is assert that under the facts of how the property was owned and the the state law that applies the property was community property rather than just owned as joint tenants with a right of survivorship (JTWROS). Whether a particular property is community property is not dependent on the title shown on the deed. Rather, most property acquired in a community property state by either spouse during the course of the marriage is community property regardless of which spouse is shown on the deed as the owner. There are some things, like property acquired by gift or inheritance, that may be separate property, however. While there might be an initial presumption that the property was simply owned JTWROS when the title states that is how the property is held, that can be overcome by showing that given the applicable facts and state law, it was community property at the time the spouse died.