Solo air forks don't have spacers - the length of the air spring shaft is a different length for different travel. Easy to change, you just need to track down the air spring you need. I think this is so the air spring piston is always in the correct position relative to the little bypass dimple that allows air into the negative spring.
If you're looking to go up to 150mm I have an air spring which I was going to use on my 2013 Revelation but didn't need it in the end.