I think WEU uses upgrades to change attack range. And you cant decrease the level of an upgrade, therefor it will run out of levels.
Lets say we got these 2 upgrades based on Long Riffles:
IncreaseRange [max lvl 100] [+10 range every level]
DecreaseRange [max lvl 100] [-10 range every level]
Every time we want a unit to increase his range we increase the level of the upgrade "IncreaseRange" by 1. Sadly we cant decrease the level again, so if we want to decrease the range, we need to increase the level of "DecreaseRange" by 1. At some time we reach the maximum level (that is 100), then this wont work anymore.
The only good way to do this, is to replace the unit with another run, but just a longer attack range