lots of useful info, thanks!
After some googling, I've found that wolf tooth components make 110bcd 5 arm NW chainrings in 34t and up (in 2t increments). Race face also make NW rings for cross, but only 38t and up.
I currently use 24/38t chainrings, with a 12 (or 13?) to 28 8-speed cassette. I think my XT rear mech will go to a 32t sprocket, not the 28 as I thought. But even so, with only 8 sprockets it's impossible to keep the same range or very close to it. with a 32 sprocket, I'd need a 28t chainring to keep the same bottom gear (and they aren't available in 110bcd anyway).
If I decide to change, I think an 11-34 9 speed cassette with a 34t chainring is the best option. More or less the same top and bottom gears as what I have now...