You know why full plate armor fell out of fashion? Why the traditional knight faded rapidly from common occupation? Plate armor is expensive. It has to be fitted to the user. Sure, loaner plate is possible, but it is not as effective. Then along comes some fellow who builds up a company of crossbowmen in a few months. Suddenly all those expensive plate armors are useless, as crossbow bolts punch straight through all that expensive armoring. 

By the way, on those relic guardsmen of yours... Are they on foot? or are they mounted troops?