Stealing Something That Later Increased in Value

In america if you stole something like bitcoin or stocks or a physical coin collection and at the time it was only worth a few dollars but later increased in value by the thousands would that change the severity of the crime? How would a statute of limitations apply? What if the thief held onto the bitcoin and sold it after the price increased? Or what if the thief never sold the coins? Does the thief have to return the exact number of coins or does he have to return the value of what he stole at the time? What if he stole something worth x amount and it later decreased in value?

You need to tell us what state. However. I'm not aware of any state that allows the charge to appreciate with the item. Similarly, you don't get a lesser charge because the item decreased in value after it was stolen (which is quite common). In most states, there's only a few levels of theft anyhow. Often the threshold for the highest is quite low (a few hundred dollars).

When you steal, you have to return what you stole. If you no longer have what you stole, you should expect to be ordered to pay restitution based upon its replacement value at the time of your sentencing.