II happily change hoses and install and swap SPGs and second stages, but somehow getting inside a reg feels to me something like say, parachute rigging. I know it's not that difficult, yet a YouTube video just doesn't feel like it's enough.
Youtube videos are not the best, I usually prefer a static web page with pictures and textual explanations. However both provide knowledge that was simply unavailable 40 years ago. So, at the time, it was MANDATORY to get some training by an expert technician, explaining tricks and procedures.
Nowadays all this information is easily available. This means that you can also do a DIY training, you do not require anymore a specific class. This is generally true for many other topics, for example learning about Nitrox or deco procedures.
Of course, you also need practice, not just theory.
For servicing regs, the best is to get an used, low cost, old style reg, employed just for becoming used to servicing it.
Once you successfully dismount, clean, reassemble with new parts and carefully tune it for a couple of times, then you can risk servicing your REAL reg...
So, albeit I answered that I would take a class (I did take it 1978 at Scubapro factory, and it was absolutely useful), probably there is no need to spend money for it nowadays, given the available audio-visual material and service manuals available online.