You can just pick one later as long as you understand that you are doing it for it's future impact and not it's current impact. Of course people who want the old license will just fork your old version as they are pretty much the same as you just re-licensed. But 5 years down the road your forking your old version won't be meaningful as your product will have evolved significantly and not be the same product anymore. Copyright licenses are long term by their nature (ridiculously long term) and their impact is best understood in those terms.