It seems there is a bug when units are upgraded.
The fault is that once you agree the upgrade the force being upgraded loses any strength whilst the upgrade is processing. So say a force has a strength value of say 10 and once upgraded it becomes 15 during the upgrade this value drops to zero = 0.
The impact of this is on recently conquered provinces. Instead of retaining a value of 10 thus safeguarding a revolt assuming 25% default morale it will lead to a 14% chance of a revolt and of course if it occurs the loss of your force to most probably your enemy.
The fault is that once you agree the upgrade the force being upgraded loses any strength whilst the upgrade is processing. So say a force has a strength value of say 10 and once upgraded it becomes 15 during the upgrade this value drops to zero = 0.
The impact of this is on recently conquered provinces. Instead of retaining a value of 10 thus safeguarding a revolt assuming 25% default morale it will lead to a 14% chance of a revolt and of course if it occurs the loss of your force to most probably your enemy.