Yes, Myth with convert to Soldiery. However many Myth you have before patch day will remain on your char, but you won't be able to gain any more Myth nor spend Myth. You will be able to speak to an NPC and convert the Myth > Soldiery at probably 4:1, so 2,000 Myth should net 500 Soldiery.
I'd not worry about the 90 gear at all unless you are having a little trouble capping tomes this week. If you are gonna earn excess Myth, buy 90 gear or buy Myth crafting mats to sell at the AH, your choice. Regardless, save up the 2k Myth for a quick boost in soldiery next week, and buy lv100 gear (which is upgradeable to 110) with soldiery. ST also drops lv100 gear, and the weekly 1 drop/week limit will probably go away with 2.4.
I'd not worry about the 90 gear at all unless you are having a little trouble capping tomes this week. If you are gonna earn excess Myth, buy 90 gear or buy Myth crafting mats to sell at the AH, your choice. Regardless, save up the 2k Myth for a quick boost in soldiery next week, and buy lv100 gear (which is upgradeable to 110) with soldiery. ST also drops lv100 gear, and the weekly 1 drop/week limit will probably go away with 2.4.