I have a Lancer that was upgraded from a Landsknecht which still retains the 'Double Plunder' promotion, however it doesn't grant any extra gold. I've noticed this while capturing a city in the save below. The Double Plunder Lancer gives just as much gold as a normal Lancer: 268.
If the promotion is meant to be disabled when upgrading (I haven't had the chance to even see if it works when still a landsknecht) then it would make this policy very bad. I miss getting cheaper roads on my way to Mercantilism which is about as far as I usually go in the commerce tree.
The save below will allow you to capture Copenhagen in the same turn.
Spoiler :
If the promotion is meant to be disabled when upgrading (I haven't had the chance to even see if it works when still a landsknecht) then it would make this policy very bad. I miss getting cheaper roads on my way to Mercantilism which is about as far as I usually go in the commerce tree.
The save below will allow you to capture Copenhagen in the same turn.