Some (most) can be fairly easy, and some can be a bear to get off. On less expensive guitars it's usually one extreme or the other. They either come off very easily because they used an adhesive that isn't too extreme, or the opposite... they used a newer epoxy to glue everything (including the pickguard, which doesn't require that strong of an adhesive because there isn't a lot of pressure on them like a neck or a brace, etc).
Better guitars (and most vintage guitars) use horsehide glue. It's strong and durable, but can often be removed with just a thin tool (like a paint-scraper, etc. I usually prefer a very thin round-ended putty-knife.). Sometimes hide glue will require some heat to loosen it. You can use an iron (turned waaaay low) on a cloth over the guard to warm it up. This has one potential problem. If you get it too warm it may also soften the glue underneath the top-wood, like the glue holding the braces on. You can instead use the iron to heat the tool itself, reheating it a few times as you work it under the pickguard a little at a time.
When using a tool like a paint-scrapper, spackling-knife, putty-knife, etc there's one thing to keep in mind. The thinner the better (obviously) and that the edge of the tool can have a bevel (an angle) on it. If it does (if it doesn't put one on it with a file), anyway... when using the tool that bevel-
angle should face down (toward the wood) as you use it like this...
======/ (direction of push)->
...instead of like this....
======\ (direction of push)->
...because having the sharper side up against the plastic makes it a little less likely that instead of just sliding between the pickguard and the wood it'll start to gouge into the wood of the top.
Myself (and Okie) would probably have no problem removing most pickguards. I've done it... geeze... lots of times. But once in awhile you'll get a hard one. If you don't feel confident about doing it, then don't. I'd still recommend a little thin wooden patch glued and clamped over the crack on the underside if you don't repair the crack itself. (The grain of the wooden patch should be perpendicular to the grain of the top for strength.) In any case, it might not get any worse than it is. And in all likelihood the buzzing you're getting isn't from the crack itself, but from another piece of wood inside the guitar under the top, like a loose brace or saddle-plate, etc. Crack edges don't usually buzz. Anyway, good luck.
Oh, b/t/w... (and this was told to me by Dan Erlewine himself, and he's sort of THE acoustic guitar repair guru) (although we disagree on a few repair techniques!) (but who-the-fukk am I?

)... anyway, Dan said that sometimes, especially on older vintage instruments, it's
the pickguard itself that has caused the crack as the plastic has shrunk over the years and that sometimes they need to be removed and reglued just because of that. (Like on an old 1949 Gibson J-45 I have where there was a small crack beginning under the pickguard for none of the more usual reasons, like loose internal-structure pieces; top-bellying; humidity issues; etc. It was increasing maybe 1/16" or so every few months. I took off the pickguard and just put it back on in the same place and now (maybe ten years later) the crack hasn't appeared again.
Enough of my rambling on. Good luck, whatever you decide.