Nice - thanks for that.
A follow-up question. Once I have the upgrade and select the new unit as my default - is there a way to upgrade existing troops or only newly raised troops will be of the new type
All existing troops of "your" type get automatically upgraded.
If you've recruited a unit from another type (IE for instance if Poland has a core on one of your provinces, and you're western tech you can recruit eastern tech units in that province) they will never upgrade. Nor will mercs.