Just think of it this way.

You can level every job up to 75, and you can switch to anyone of them through your mog house or through nomad moogles. But, you must have a subjob for that.

Say you have PLD at 75 and WAR at 50 or so. If you made WAR your subjob for PLD, it will turn out as PLD75/WAR37, simply because your subjob will be made half of your mainjob. You can't use any abilities higher than WAR37.

But if you switched WAR as your main, it will go back to level 50, and just for teaching sake, you wanted PLD to be the subjob, it will go down to 25. Because that's half of the mainjob.

You still keep all your levels on each job though, they don't get cut off or anything from subbing them.