For example, if I send a card as Polish that buffs my Winged Hussars, will my Livonian Knight Brothers benefit from these buffs as well or no?
Same question if I, say, buff crossbows as Germans then convert them into Landwehrs.
For now assume the only cards that carry over are those that affect generically like cav combat
in general they are separate units and are not affected by the same cards.
unless the devs added shadow tech to cover them which you dont know how that will work and whether it works
Usually, buff cards specify the type of unit it affects. Buff cards that affect a wider range of units usually specify the unit tag. Something like “15% attack to light cavalry” or “10% speed to artillery”. Team cards often do this.
Take a look at this Dutch HCC:
Infantry Combat - Pikemen, Halberdiers, Skirmishers, Revolutionary Sharpshooters, and Musketeers get +15% hit points and attack
This card does not buff all infantry, only those units listed. It will not buff Mercenaries, Outlaws, Native, or replacement units that are tagged as infantry. This card is fairly unique in that affects a revolutionary unit: the Revolutionary Sharpshooters. However, it will not affect actual Revolutionaries (the musket unit all settlers get changed to at the beginning of the revolt).
By contrast…
Cavalry Combat - Cavalry get +15% hit points and attack
This card will buff all units tagged as cavalry including mercenaries, natives, outlaws, and replacement units. This would make it very valuable to send before revolting.