"Use" means to convert it to a different form
...and to extract work while doing so. No? Which means converting it to some form where it has higher entropy.
Look, it seems like you are more or less just making stuff up based on your rather incomplete understanding of thermodynamics, which is why recommended you to actually read up on the subject. Setting aside the rather dubious claim that a perfectly isolated system can even exist, what you are describing is exactly how you could define a perpetual motion machine of the second kind: an isolated system that performs work using energy from a single heat reservoir, without transferring heat to an external cooler reservoir. Such a machine cannot exist, because it violates the second law of thermodynamics, which states that the entropy of an isolated(!) system never decreases. The system tends towards thermal equilibrium, where all the energy is converted to a uniform distribution of heat.
If you had bothered to look any of this up, you would have already known this, instead of speaking out of your ass based on what you think a perpetual motion machine is and why it must be impossible.
The key flaw in your reasoning is that you seem to think of energy as something that's equivalent regardless of its form. It is not so. In fact, whenever we use energy to perform some work, it actually isn't the energy in itself that we are using, but its state of being far from equilibrium. Its "order", for the lack of a better word. The energy is just a carrier. And when we are using heat energy, we are actually not using the heat in itself, but exploiting the temperature differential between the heat reservoir and a cooler reservoir. That's why all heat producing power plants need cooling water, and the reason jet engines get higher efficiency when flown through cooler air at high altitude (even if it's thinner). Conversely, it explains why a refrigerator requires external energy even though it is removing energy from its interior, and why a heat pump can have more than 100% heating efficiency whereas distributed heating can never reach 100%.
One physicist who have written a lot about these things is Ilya Prigogine, if you are interested to read more, although I'm sure you could find many others.