Upgrade my rooted S4 sgh-i337 to 5.0 from 4.4.2, using a Mac

Hi, I need xda's solid information to upgrade my rooted S4 sgh-i337 to 5.0 from 4.4.2. I'm a Mac user, so I haven't been able to find anything useful so far. It was an unlocked AT&T phone, now being used with T-Mobile. I have already downloaded the 5.0 update.zip from AndroidFileHost onto my Mac. OK, here's a stupid question: Why couldn't I simply do a factory reset (in which case would I lose the root?) and then update from Recovery Mode ["apply update from external source"], and re-root later? Thanks for reading, and for whatever help you can give. Oh, I'm still learning, so please don't assume I know anything.:confused: