We will freeze-dry any milk that you would like to save. It's your milk, and your choice! However, we advise following the CDC recommendation of 12 months max freezer time from a safety standpoint. For nutrient preservation however, the two biggest things that affect the decrease in nutritional content of breast milk are the length of time that it is frozen, and the thawing process.
For the length of time component - the sooner you use frozen milk or freeze-dry it, the better. Most milk banks discard any unused milk after 3 months for this reason. So when deciding on whether to send the oldest or the newest milk in your freezer, if you send the older milk, you can stop the clock on any further nutrient degradation. Then, if you like the service and the result, you would still have the option of freeze-drying your fresher milk in the future if you wanted!
For the thawing component - the freeze-drying process actually requires the milk to be frozen as the first step - but it is never thawed. The process that removes the water is called sublimation - so the water goes straight from solid (ice) to gas (water vapor), bypassing the thawing step. This is important because it is the thawing step that can cause many changes to the quality of the milk. So while the freeze-dry process requires the milk to be frozen initially, it prevents some of the changes in quality, taste, and nutritional degradation that happens when milk is thawed from a frozen state.