Point 5 of the army lists book states :
Any UGs can be downgraded by one quality grade and/or by one shooting skill (Skilled to Experienced or Experienced to Unskilled)
representing less strong, tired, or understrength troops. If any bases are downgraded all in the UG must be downgraded. So Average-
Experienced skirmishers can always be downgraded to Poor-Unskilled.
My reading is that if you use a mixed TUG, typically front ranks spearmen, rear rank bow/crossbow ..if you downgrade the shooting skill of the bows, you only affect the shooters but if you downgrade the quality of the same shooters, usually from average to poor, you must also downgrade the spearmen from average to poor.
Am I correct ? ( am I clear enough )
Correct.