As i have said before, there isn't a Yoda surprise. It was a surprise meant for Luke and not the audience. If it was meant to be a surprise then they wouldn't have used him in the promotional material prior to the films release.

I've seen fans that try to explain Yoda's crazy behaviour when we first see him as it being down to him living secluded so long on Dagobah. But this is completely wrong. He is only acting that way to fool Luke. It was more of a test to see if Luke was open minded enough to think that this little green goblin could be the greatest jedi master. he failed.

So there really isn't a problem with having Yoda in the prequels. No surprises spoilt.