Those conditions are the one you mention, secession if it so chooses, and the right to divide itself into as many as five separate states. No one has seriously suggested that Texas undertake a self-division. However, its Senators would increase from two to ten, were it to make this change. And it could be done by action of the Texas Legislature.
Congressman Billybob
Well, no, it would require the consent of Congress as well. "Admission conditions" or no, Texas' entry into the union did not override the Constitution. It had to agree to the Constitution in order for it to have been lawfully admitted in the first place, and its lawmakers and officers all have to take oaths supporting the same.