Are they easy to replace? No, not really. They're not "designed" to be replaced, but it is possible, like most things. Not too hard to pick up a 2nd hand instrument off ebay. Louigi swopped glass for me, frankly, it's not a job I fancied tackling myself. Of course it's doable though, or folk wouldn't be selling those after-market speedo and tacho backing sheets.
Have to warn you though, after I got mine swopped, the new glass cracked in exactly the same place shortly afterwards, so prepare yourself for that.
In my case, because I'm proud of my high-mileage reading, I'm really keen to keep the original instrument.