I bought a jump potion for WAR because I already had PLD and DRK above 50 and I didn't want to grind through lower level dungeons with 2 skills, and had absolutely no idea what I was doing with all the skills that got thrown at me. Which kind of makes me wonder why new jobs are added at 50 or 60, because people are already bad when they had all that time to learn them, and now they're bad right at the start.

Anyway, I learned it by doing stuff unsynced with my partner for other stuff, since no one cared if I had my rotation perfect and I had time to figure it all out. But when ShB comes out I'll be picking up GNB, and I plan on going back through leveling dungeons to learn the skills and rotations as if they were learned normally.

But if it's a job already in the game and you're worried about the changes being made, from what I've seen none of them completely alter the jobs, so you're better off learning them normally now and then adjust when the changes come.