Every human life has value. There is no instance where I would value the life of a frog over the life of a human being. By logical consequence, I should always prefer one more human over one more frog. QED.
You say that because you're a human. If you were a frog, I bet you'd value frogs' lives more than humans. Why do you think it's your place to change others' lives into whatever form you personally prefer?
My point is that it would be vastly more cruel to turn a frog into a human, let them live a little and then abandon them to fend for themselves after they've outlived their usefulness to me (which is how I understood your point above) than to leave the frog alone entirely. By turning them into a human you have acquired a degree of responsibility for their well being which I would argue you can't shirk under normal circumstances. (C.f. making babies.)
This logic hinges on the assumptions that 1) humans have a higher state of consciousness than animals (which might be debatable in certain cases, ok) and 2) that a frog is indifferent towards its current state of existence because it doesn't know any better.
Lastly, you write "every human life has value" but in that thought experiment in the OP, they are not humans so far so that logic doesn't hold. Just because you have the ability to elevate/create life of a higher consciousness, absolutely does not mean that you have to do so IMO and that it can be downright inhumane to do so. (Again, babies, contraception, abortions etc.)