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I think also that at a certain year, the ability is lost. Not sure though.
 
Followers of the Norse religion (because Christian Norse cannot) will forever retain their ability to traverse rivers whether old or reformed, but they cannot go through the upgraded provinces of other kingdoms. But, if that land belongs to you then you can freely traverse these river highways as you wish. What does that mean? It means if you conquer Russia in a Great Holy War you now have permanent use of it's river systems. This is what makes Viking Empires so formidable - you can deliver an entire army of 100k into the middle of Cumania to counter an attacking Mongol Horde whereas with any other religion you would have no chance of amassing sufficient forces to meet them even if you had them.