Later, yes, but only if the owner of the county allows it. 

• All counties that are owned by Medieval Empires will not allow players of other factions. 
• In the future, owners of counties will be able to decide which faction they allow in addition to the faction that the county belongs to.
• If a player changes factions, while being on a county that does not allow other factions, they will get evicted. 
• County owners are always able to join their county, no matter their faction.