In the Player's Guide on page 10 under 3.6.1 Attack, it says:
"If the Formation is not fully deployed when the Attack [A] activates, it may remain stationary during the Movement phase and Deploy until fully deployed."
On page 25 under 4.1.1 Cavalry Conversions, it says:
"cavalry is immediately moved forward to contact with the target"
There are a couple of possible interpretations here:
a) under an Order Conversion the cavalry doesn't need to "immediately" move forward if it has undeployed units. b) under an Order Conversion the cavalry must immediately move its deployed units into contact
c) a cavalry formation with undeployed units cannot use the Order Conversion to change to Attack
-Yahoo Group, 4/22/18
The intent was that the cavalry Formation (all of it) will move immediately. If one doesn’t want to go, because the Formation is still played or mostly ployed — well, the player doesn’t have to choose to convert, so if they don’t want to, they should not.
-David