How hard is it to change MOS in the National Guard?

Most states are more than willing to add reclass as a condition for your extension.
Tell your AGR and your chain of command, use a real recruiter (RRNCO) who will find a vacancy for you to transfer and reclass for your extension.
Your extension window starts 365 days before your ETS, and the sooner you start this, the better the results.

Not that difficult, you can reenlist for it or you can find a unit with a slot for your grade in the MOS you want and start a transfer. The Guard usually has MOS re-class Schools that you can do on AT orders.

In the Reserve and Guard system it's super easy. Even easier if you go to the unit/units you are interested in joining and ask what vacancies they have. If it what you want and the unit is interested ask to fill their vacancy, then its just a matter or transferring to that unit. This can usually be done in a 1-3 month (Even easier and faster if you change your residency to a location close to the gaining unit (You can start split training with the new unit to speed the process up as well)). The unit will get you slotted for MOS Training. 11B Guard units almost always have slots that need filling. The only caveat is if you change states, that the losing state may slow walk your transfer packet in order to keep their numbers up in the state for a while. Baring a hiccup or unforeseen event it's seamless to transfer within a state. But if you move close to that unit, start drilling with them as a split trainee; you'll still get paid, start gaining experience, encumber the slot so no one else can fill it, and the new/gaining unit will pull harder to get you officially transferred to them.

I've held 4 MOS' and 2 identifiers in 34 years (11B, 21B, 37A and 35D). Two of which I obtained by transferring to a vacant position within the unit I wanted to join. One thing to consider (More so as an officer than an enlisted (because it's frond upon for Officer to change MOS')) will this move slow down your career? As an enlisted 91B to 11B I seriously doubt it. Moving slowed me down at times, but I held the jobs I wanted, not the ones forced on me, so I was happy with my changes. I've enjoyed my 34 years of service (Active, Reserve, and Guard) and wouldn't change much of what I did. A few small things here and there, because we all make mistakes, but overall I do it all again.