Wash it, then sterilize it. Baking in the oven at about 200 for 2 hours or until the center of all mixes are at at least 180 will do that. Tedious, I know, but that way you won't be carrying any potential diseases or pests from one plant to another, and a pest that didn't bother plant A might be problematic for plant B.

Yeah, organic bits might be a little...um...scented in there.

You can then (or simply, if you'd rather not sterilize) mix old with new as well as you can.

Or, if repotting singly, just mix old mix with new mix and repot that way, cleaning it out as well as you can as you go, and washing out the old mix to get rid of the inevitable decayed bits of old root, fallen leaves that we all miss, and so on, and replacing with fresh material. The plant's not going to get any new pests as it's not in anybody else's material. Just clean your tools with 1:10 bleach water between plants, and wash your hands.

I tend to discard old mix as contaminated, but understand that renewing 30 containers at once might be a bit of an effort and expense...

It was a big chore to find all of the gritty-mix ingredients in my area and they were quite costly. Also, mixing the ingredients for I think it was more like 50 containers was exhausing.

Understood. :-) In your case, I'd keep them, sterilize if you must, or remix per pot if that's easier for you, cleaning what and if you can.

Gotcha, that changed. So the fine bark rotted away. You can remove the plants when the time comes (for you, now should be fine because do they ever really stop growing?) and clean them out, remix partially with new to refresh, and replant.

Mixes really should be refreshed every now and again, so this should take care of it.