I was Viking this season and as we all know Vikings won in the tournament in the S5 reveal stream so the question is if I switch back to samurai will I lose the rewards for the Viking victory? I was samurai since beta and I missed them so I wanna go back
You should be fine to switch since they took a snapshot of all the members of the Vikings one hour prior to the stream starting. To be on the safe side though, I would wait to switch until Tuesday. If you haven't received the reward yet, they should be fully delivered by next Tuesday.