You're right that it would need a true legislative change, but not that it's sacrosanct in the constitution.
It's part of Texas law in our education code found here:
https://statutes.capitol.texas.gov/docs/ED/htm/ED.101.htmThis is the code that lays out the legal basis of our university and how it runs in a basic way. This code is what has to be changed in the legislature in January for us to join the UT system. UT will be drafting the proposed legislation and it will just take a majority vote for us to be part of their system.
Whether the part about our name never changing survives the new legislation about SFA is anyone's guess at this point, but I'm sure current SFA regents will try to keep it from happening. (I honestly don't know if this whole section will be amended or essentially moved to the part of the code that deals with UT.) But, this code is changing soon and the name provision potentially (I doubt it will) be changed with it.
In the future, a board of regents couldn't change our name by a vote. It again would have to be changed by law.