"Substantially less favorable" is statutorily defined as "the wages, hours, or other conditions of the work offered are substantially less favorable to the individual than those prevailing for similar work in the locality". Tex. Lab. Code, Sec. 207(b)(2). A theoretical scenario in which the predecessor earned substantially more than the employee might be the start of a case for the lower wages making the new position "substantially less favorable," but unless the job is somehow unique to that employer a proper analysis will look at what other employers in the locality are paying for similar work.
You don't have to be insubordinate to lose unemployment eligibility over refusing a promotion, absent very unusual circumstances such as the one to which you allude.

