Mushy backbulb on Oncidium

The brown part of the backbulb feels hollow and mushy

     The oldest pseudobulb on my Wilsonara Pacific Perspective is rotting away.  The brownish portion of the pseudobulb (whose color the camera flash so nicely accentuates) is hollow and mushy on the inside.

    I don't want to repot the orchid now, since it's in full bloom, and I don't want to risk losing the flowers early.  The roots seem healthy, so I hope I am not making a big mistake by waiting.  But as soon as a blooms fade, I will be repotting, and cutting off the rotten bulb.  I will also be switching to a different potting medium, perhaps one specifically formulated for oncidiums.  Currently I have it potted in paph&phrag mix, because it's a fine-grade mix and the oncidium's roots are thin.  But the rot seems to indicate that it needs a change.