How do the research upgrades work?

[Pavelenkov]

Specifically, when I upgrade knights for example, does the upgrade effect the knights I currently have? Or only the ones I recruit after the upgrade is done? And if I revoke research down to zero, for let's say Sentries, what happens to the sentries I already have? Thanks.

[Tooltip]

Research at a single city effects all troops belonging to that city. However...

When supporting another city, troops use the research that city has and upgrade/downgrade levels to match.

Level zero research is the same as level one, however you cannot build new units.