I can't answer that. The Dhamma is the truth. The Dhamma is also the Buddha's teachings. If we are talking about the Dhamma as the Buddha's teachings then the texts say yes, it is impermanent. There will come a time, in the future, when the Buddha's teachings disappear.
Today the teachings are not as pure as they were when the Buddha was alive, that's only natural. When he taught it was absolutely pure. It was right from his mind and from his voice and everything was pure.
People tried to remember what he said, they told other people what he said and later they wrote it down. Then other people wrote it down. People translated it. Then it was translated again and again.
What we have today is not going to be perfect. There are going to be mistakes. And there are some obvious mistakes that can be found in the scriptures, not many, but some that seem obvious. They predict that in the future, in another two and a half thousand or maybe more years, the teachings of the Buddha will get so twisted and warped that there won't be anything that is actually truly from the Buddha.
So the Dhamma, in terms of the Buddha's teachings, yes, that's impermanent. It will disappear. And it's because it disappears, that another Buddha has to discover it and the cycle goes again and again and again.
The Dhamma as the truth! Well, the truth always exists whether it is written down, whether we know about it, whether we agree with it, whether we like it. It doesn't even matter. The truth stays the truth. Right now the sun is shining somewhere, it's cloudy here, but it's shining above the clouds. We know it's up there above the clouds, we feel absolutely certain it's up there above the clouds. It's the truth, the sun is up there. But we can't see it, can we? If we were blind we never know for sure the sun is up there or not. But even if we are blind it won't stop the fact that if the sun is up there, it is up there.
So with the Dhamma as the truth, that's always going to be there.
Now the reason I said, "I can't answer", even though seemingly I did is because, what happens when there aren't any human beings left any more? Does then the Dhamma exist? Because the Dhamma is the truth that applies to, not just human beings, but all living beings. So if there was a time when all living beings disappeared, does that mean the Dhamma disappears, too? I can't answer that. That's beyond even my logical understanding.
But as to the Dhamma of the Buddha, yes, the teachings of the Buddha are impermanent. However, as to the Dhamma as the truth, it seems it's always going to be there.