I got into growing before I knew how to repot too! You should have seen me the first time I repotted anything! So nervous! And just the other day I did my first paph and it was nerve-wracking thinking about it, but ended up being really a fun and enjoyable experience. I can't wait to repot the next! It also is nice because you can check on the status of the plant's roots and the healthiness of the plant.
Now, that all being said, I am afraid I am not the right person to answer your question about sphag and dendrobiums as I only have 1 dend. which is in a medium consisting of large coconut husk chunks. I repotted a phal that was originally in sphag, into another medium, as I had problems with the sphag being packed in to tight, so the roots rotted. So, the one thing I can lend a hand on is make sure that the sphag is not packed too tightly.
I'm certain someone w/ more dend. specific info will help you out! Glad you are with us here!