repotting Catasetum, please help!

[chemist]

So over the weekend I bought Catasetum Tenebrosum.
From conversation with seller i understood this plant was repotted recently, how recently i don't know, but it has new roots about 2 inches long bound to the media, so maybe month -two ago?
It was potted (today i removed it from media) in sponge rock (I'm not sure about name for this material, please see photo).
So I noticed that older roots not drying at all (close to surface) even though last watering was on Saturday. So i decided to take a look at the roots despite that plant is in spike.
Here are pictures:


To me those older roots from old PBs looks rotten. Does it make sense to repot it with old roots (like seller did or maybe they become rotten after repotting) or better remove it?
It came in 3" round pot. But if i remove old rotten roots the pot will be too big for root mass in my opinion. So, do I have to pot it in smaller pot?
Also do I need to change media to bark (i gained control over bark) or pot it in the media it came with? I also have LECA if its better for Catasetums.

Thank you in advance for advise!