I grew Neps & orchids from 2000 or so until winter 2006/2007 when my dad got sick - I sold all my plants cos I was spending all my time taking care of him and was never home to take care of my plants. I probably used cypress mulch half of that time. Do not use cocoa shell husk mulch (smells like chocolate) it turns all fungusy really quick. Look for cypress or cedar. I like shredded cos normal mulch is pretty big chips but the shredded is "finer" but still pretty bulky. I'm getting back into the hobby now and will be using the cypress mulch again when I get my new vivarium wall project setup. Soil in the display tanks and most pots in the GH tanks will be a shredded LFS, mulch and turkey grit blend used over a drained false bottom so I can set the misters to rain whenever and not have to worry about flooding the soil.
I can't recall how the plants "took it" but anytime you disturb Neps roots you're going to set it back a month or two before it gets going again. For gentler transplanting I generally just removed the excess LFS and left the main root ball of LFS the way they were bound. Simply pot them on in the new mix and let them discover it by themselves as they grow. If you see most HL Nep pics in the wild they're growing in moss, twigs, dead leaves, etc. nothing too "fine" or compact.
I should add I grew in climate controlled conditions of 80% RH and proper temps, air flow, etc. In a windowsill this soil mix may be too airy and the plants may dry out too quickly, I don't know cos my windows won't grow much more than ferns and begonias all my Neps and other high light plants have to be in grow chambers.