I have cellulose insulation that is R19 in the attic from 1985. Can I just add more of it without removing the old stuff? I live at 5,000 feet and have three seasons: Cold & some snow, temperate spring & fall, and hot summers (up to 105 degrees Fahrenheit). I am not fond of fiberglass for many reasons, and that is what most companies suggest I use.
You certainly can add more cellulose, but only if the existing cellulose if in good condition - that is, it's still fluffy and isn't sagging.
If this is the case, you could add more cellulose OR add unfaced fiberglass batts. Lay the batts on top of and perpendicular to the floor joists in the attic, edge to edge. The additional blanket will improve the attic's insulation. You can do that with up to 10-inch batts which when combined with the existing 6 inches of cellulose would give you R49.
I don't know how strong your opposition is to fiberglass, but if you're willing, give it some thought. It's good stuff, the few the drawbacks being that you lose storage space (you can't put stuff on top of it) and you must make sure you have attic on attic insulation, because fiberglass gets moist in the winter, and ventilation is needed to dry it out.