I would wait until new growths are just starting to show nubs of roots.
Personally if you are moving to S/H I wouldn't wait until they are an inch long as they will have already started tailoring themselves to the current medium and the S/H is quite a change, so you risk loosing the new roots, however if they are just nubs I find they transfer well into S/H really well.
I would wait until the growths are big enough to be starting to show root growth though, this should give the shortest time between root loss from repotting and from transfer to S/H, and the new roots getting established.
I like growing Oncidiums in S/H, they do the best for me of all the ones I've tried in S/H. I have a Sharry Baby I plan to put in S/H and all my other oncidium types are now potted that way.
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