When a solder is augmented and becomes a MEC trooper, they lose their old skills (except for the single trait based on what class they were before), but do they lose their rank as well? If I augment a Colonel, will I be able to select all seven new MEC Trooper abilities for them right away, or will they revert to Squaddie?
Yes, you maintain rank when you change class, allowing you to pick abilities up to your previous rank. You also maintain bonus Hitpoints, Will, and Aim; This means that augmenting a Sniper will result in a MEC with higher accuracy, but less health.
Once augmented, however, they will begin to use MEC progression instead (same as Heavy).