There's a short guide on Open Advancement on the wiki here. You can change to any other class once you're level 101+ and have completed the 4th job advancement, and change back at any time. Stat points for all warrior type classes want to go all into strength, secondary stat requirements were removed a very long time ago, and there's an auto assign button now to assign your stat points.It's unlikely you were playing during the release of the RED update, but they added in a new system called 'Open Advancement' for the explorer type classes.So for example I have advanced to Fighter, then advanced more to Crusader. I can switch to Paladin when I'm about to commence my 4th job advancement?
What this means is that once you reach level 101 or higher, you have the option to pay a fee (in mesos) to change your job to one of the others in your class type. So for example if you were to level a Spearman and decide you don't like Dark Knight when you get to it at 4th job, you could change your job to Paladin or Hero whenever you want.
This works for all explorer type classes, but you can only change within your basic class, so for example magicians can only change to other magician explorer jobs. The cost of changing goes up each time you change and the higher level you are, but it's a good way to try our different classes if you decide you don't like your current one or want a change.
Another thing, where can I find class guides? Like the correct skill and stat points to distribute to and all? Thanks!