First, wait for new roots to appear. They will be the ones to adapt to the new medium. So no rush! It may be a few months before those new roots start... and note that I said "roots" not "growth". Sometimes new growth and new roots go together, sometimes not. It's all about roots. Repot when the root tips just start to peek out - a couple of millimeters, or even catch it when it's just little bumps. Those new roots are fragile, you need to avoid any damage to them. But if you catch just as the rooting begins, if one or two get damaged, there will be more right away.
When you do repot, if it were my plant I'd cut the pot, remove the old medium and pot bits that come off easily and leave the rest. It looks like this was potted in sphagnum... for Catts I would lean toward medium bark for a plant of this size, in a pot that will give it room for 2-3 years' growth.
I don't try to remove all the old medium from any orchid that I'm repotting. Preserving roots is soooo important. Rigorous cleaning is going to damage roots. Don't worry about the old bits, just rinse well to remove potential rot, etc. but then just pot it up.
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