I think Stake.com has signed a contract with Watford, so they will still have to honor that agreement..even if Watford was relegated. I watched Eddie's (co-owner of Stake) streaming session a few weeks ago and he said that they will just have two sponsorship deals, if Watford was relegated.
They will just sign a deal with another team and still honor their contract with Watford, even if they are not playing in the Premier league. (The new deal might just include a clause to address the issue, if that new team gets relegated)
I believe that they will not have to focus on another one right away, I mean they may in the future, but Watford could comeback as well, they did that very recently. They had sportsbet.io sponsorship, on the first year they relegated and on the second year they came back, on third year stake got it, and now it could end with relegation and fourth year could be coming back again.
Watford has been going up and down for the past 10 years, they are good enough to be in playoffs at championship at any given season, but bad enough to not be comfortably in the premier league so this causes them to be an elevator team who goes up and down constantly. If stake waits enough, they will be back in premier league I am sure.