Retraining is not possible in STW - however, when u have damaged or understrength units - you can combine them if they are the same type and would not exceed the maximum number of men per unit after combination. This does not allow for retraining per se - but can keep veteren units alive by merging them. Or it can provide some boost to fresh, low honor damaged troops.
Qapla!

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BSM_Skkzarg
"A mind is a terrible thing to taste."