Thieves don't need to job change at job 50, although it will benefit you so you can max out skills like venom dust and enchant poison but it isn't a must. Job level 45 is good or 44-40..
Unless you can find a nice friend to leech you, or lend you dopples, fire enchant you and tank you at Mi Gaos (it's how i help my friends get to job 50 XD) or if you already have a high level char then dual-client and tank yourself..