There has been some confusion on what the erratum for the Light Cavalry move (LCM) means.
What this erratum does is move the LCM from AFTER the activation to IN the activation. As a consequence, only the models that have not yet ended their activation can perform the move. This brings LCM and Reform to the same mechanism.P. 84. Light Cavalry
Replace the first sentence of the second paragraph with:
After an independent light cavalry model completes its action, the
model can advance up to 5˝. After all the models in a light cavalry
unit have completed their actions, each can advance up to 5˝.
A running model can't "complete its action". If you run you do not have an action. If you do not have one, you can't take it nor forfeit it, so you can't complete it. Same would happen to a model which fails a charge.
If at least one model in the unit takes or forfeits its action, the other models may use reform/LCM (except those who ran or failed a charge or anything similar). This last sentence is telling that you don't need to have each and every trooper completing or forfeiting an action to trigger Reform/LCM.
Light Cavalry unit with troopers A, B and C.
All make a full advance and then make their action. Then they all get to make their LCM.
All run. None of them gets to make their LCM.
They get the charge order. A runs, B fails the charge, C makes a successful charge. Then C gets to make the LCM, A and B cannot make the LCM.
A is knocked down and forfeits action to stand up. They get the charge order.
Due to this
A makes a full advance and then ends its activation. B fails the charge, C runs. None of them gets to make their LCM.Originally Posted by Prime, p. 46
The same basic principles apply to the Reform ability (Kossites, Striders with UA, etc.).