Not sure if this has already been answered, but the answer is; they don't have to. Contractually the maximum UAX block hour ratio is 120% to mainline narrowbody block hours. Before this JCBA was ratified the actual uax block hour ratio was somewhere south of that. For instance I remember hearing it was 112%. So technically they can add 8% more UAX flying. A reduction in UAX flying will occur when or "if" UA decides to get more Big RJs or Qs.