Before solo air, it was very easy to change fork travel on the dual air forks using all travel spacers.

With solo air forks, this approach is no longer possible. If you put a spacer under the air piston head, the fork will never be able to equilibrate the positive and negative air chambers. Inside the air spring stanchion of solo air forks, there is a small groove/dimple that allows the positive and negative air chambers to equilibrate when the fork is at a full extension. Once the fork compresses slightly, the air piston head and seal moves past this dimple and the air springs can work like normal.

As a result, if you want to change the travel of your solo air fork (Pike, Revelation, Reba or SID), you will need to buy a new air spring (or at least the air spring rod). If you're handy, it is possible to cut down the air rod by around 10mm and still retain the original threads. If you're not handy, or you want to reduce travel by more than 10mm, then you need to order a new air spring.