not 100% sure since patch but wherever you buy the boas there should be another set of the same boas further back on the page that
requires the old boa + gold to upgrade to the 1-85 version.

The boa will work up to the max level it has. if it's 1-80, you'll get the exp bonus till 80, just like if it's 1-85, you'll get it to 85.

meh

It does give xp bonus till you HIT 81 though

Since you dont lose the stats from the gear when youre 80, the exp and stats count until 81. You can upgrade the items to 1-85 @ the vendor in og. (Yes ofc they work until 86 then.)