Your Den looks in good condition so you could go ahead and repot it now in s/h. I repot my Dens anytime during the year into s/h with no adverse results so far. Since this is a Phal type (probably), it will not need the winter rest for blooming.
When transferring to s/h, I would make sure to come as close to the pot size it is in now as I have discovered that my Dens bloom better in s/h when they are tight in their pots.
I find the Phal type Dens prefer more humidity and moisture so I allow a little more of a reservoir at the bottom. I also discovered last year that the s/h makes the pots a little cooler so if you are growing in a greenhouse or converted shadehouse or outside (if you are lucky), keep that in mind so you don't let the roots get cold and wet. When my converted shadehouse drops in the mid 50's, I keep my s/h a little dryer. Once the temps go up and the heaters are able to keep the temps in the 60s, then I water again.
The size pot I would pick for your Den would be a recycled 20oz soda or powerade bottle. Holes at about an 1" to 1" 1/4" up.
I use Hydroton mixed with Arcolite (available locally in bulk) which I really like. The Hydroton is round and fired, while the Arcolite is a rough pebble natural to Arkansas (as far as I know). The Hydroton allows for more air flow. The Arcolite keeps the roots nice and tight if the plant is top heavy.
I find that the Phal types like lots of light. I put mine in the same area of my SHouse with the Catts. They are super easy to grow and are reliable bloomers.
Good luck. Sorry so long.