From moss to LECA
 

I have many Cattleya alliance plants in sphagnum in which I get excellent growth. Roots so thick they push the plant up and out of the pots.
However, I do not grow these plants in anything larger than 4 inch size. I would like to transfer to LECA or 3/4 inch rock to continue on to specimen size. Getting any or all of the moss off the roots would be nearly impossible without destroying the root system and thereby setting the plants back significantly.
The obvious method would also be the easiest: set the plant undisturbed in the LECA. Has anyone ever had to do this and have you had or would you forsee any problems with this? Any advice is appreciated.

From moss to LECA
 

Good luck!

I use either LECA or lava rock for all my orchids. I always remove all the old medium (bark or moss) before transferring them to the LECA or lava rock as, in my humid and sometimes cool environment, roots will rot if the old medium remains. I soak the orchid for a while and then just set aside a nice chunk of time and carefully remove the moss/bark. It takes quite a while, one of the reasons I would never use bark or moss again for potting up my orchids. I usually select a basket pot that will hold the orchid for many years to come.

The only thing with LECA and red lava rock is that, as you are not changing the medium every year, you have to periodically flush the pots very, very well to get rid of the fertilizer build up.

If you soak your rootball in water for 5 min, you will usually be able to pull out the majority of the old spaghnum. At that point you can do as you wish.

I disagree with the need to remove old moss. Use large lava rock (LECA is so over-rated, and too lightweight for catts) and leave enough air space for the moss to dry out. Yes, the roots in the centre will eventually rot and decompose, but by that time new roots adapted to new conditions should be keeping the plants alive.

The few catts that I still have in pots do well in lava rock, but they didn't like LECA.

Hold the plant upside down and blast it up close with the hose. It sounds rough but is gentler than pulling / tearing etc.

I usually just throw my fast growing Catts in a new pot without pulling them out of the old one if the media is still good, just like you suggested. I use bark though. Moss can sour pretty rapidly, but if your plant is vigorous enough to grow in the new pot relatively quickly, the back roots aren't as important. However, having rotted out back roots can make the plant unstable and can breed unwanted pests, damaging fungus or damaging bacteria. If you can keep the old spag relatively dry, then I'd think you could pot up without rotting the roots. If the spag stays wet for long periods, you might want to remove the old media.