actually we seen something similar and evolutionists guys said that this prove that creationist are wrong. so in such a case evolutionists will claims that the fact that it happen prove evolution and not disprove it..
What?
lets test it. how much time we need to wait and see if evolution can happen or not? plus: see my signature link to understand why the burden of proof is on the evolution side.
If you're talking about a creature changing into a very different creature, that could take many millions of years, depending on the situation.
Let me offer an oversimplified example. Let's say we have a critter called a cabbit. Let's label our starting species cabbit0. After many generations of mutation and selection, we get a new species, which we'll call cabbit1. Now cabbit1 still looks basically like a cabbit, but it can't breed with cabbit0. We don't have a very different creature by your standards.
But much later, we get another speciation from cabbit1, cabbit2. And the process continues.
Cabbit1 is a little different from cabbit0.
Cabbit2 is a little different from cabbit1, and somewhat different from cabbit0.
Cabbit3 is a little different from cabbit2, somewhat different from cabbit1, and quite different from cabbit0.
Cabbit4 is a little different from cabbit3, somewhat different from cabbit2, quite different from cabbit1, and very different from cabbit0.
Cabbit5 is a little different from cabbit4, somewhat different from cabbit3, quite different from cabbit2, very different from cabbit0, and, well, it's so different from cabbit0 that we really shouldn't call it a cabbit any more. Let's call it a Cabello. So now after millions of years and countless generations, we have something seriously different.
But if you look at it, we could rename the previous generations. Cabello could be cabello0, cabbit4 could be cabello1, cabbit3 could be cabello2, cabbit2 could be cabello3, cabbit1 could be cabello4, and cabbit0... isn't a cabello, so it's still cabbit0.
In other words, if you want more than simple macroevolution (speciation), you just have to wait (a lot) longer.