We do it every day in our shoe repair. But there is a limit to how much can be added or lowered. Every shoe has a balance built in to how the shoe was lasted or in other words, designed. We find that most shoes, from women's high heeled pumps to cowboy boots to can be raised or lowered up to 1/2 inch with no problem. Some can be raised or lowered as much as 3/4 inch or even 1 inch. Balance of the shoe determines how high or low. There is good way to see how much can be adjusted. Hold the shoe or boot on an edge of a table so that the sole is on the table the heel is off of the table. Next raise the heel above or below the table as much as you want to adjust the height. This gives you the ability to see how the balance is affected. Does the toe raise or lower too much? It takes a trained eye to tell for sure, but this technique will give you an idea of how much a heel can be adjusted.
What if you have a short limb and need to adjust the height?
This is totally different than the answer above. In the case of a short limb a shoe can be raised as much as your shorter leg requires. We've raised anywhere from 1/8" to 6" or more. Balance is still an issue and the sole thickness will need to be adjusted in proportion to heel height.