At any time spouses can agree that all or part of their separate property be converted to community property Tex. Fam. Code Sec 4.202. 

This agreement however must be in writing, signed by the spouse and identify the property that is being converted.

The transfer of separate property to the name of the other spouse does not constitute a conversion agreement.

Property purchased partially with separate funds and partially with community funds is a split title.

So Generally whether property is separate of community is determined by its character at inception.

A court MAY NOT take the title of a spouse’s separate property by awarding the property to the other spouse.

However, a court can order that the separate property be sold to support a minor child.

Commingling occurs when separate and community property is mixed together.  If the property in a financial account are commingled so as to make it impossible to segregate then all of it is considered community property.

The spouse claiming certain property as separate must trace the property to be separate.  This means establishing the separate origin of the property through evidence showing the time and means by which the spouse originally obtained possession of the property.

It is established in law that testimony that property was possessed at the time of marriage is not enough evidence without documentation.

Statement of accounts, dates of transfer, amounts transferred in or out and sources of funds is necessary to show tracing of funds put into brokerage accounts. (Tracing commingled brokerage accounts is a huge undertaking).