using secondary class change to learn different skills

i dont feel like starting an entire new char over to get some new skills so should i be able to change my class and do missions over to learn some of my new class skills?

You can redo missions, but not quests. Likewise, you can't earn mission rewards more than once per character.

There are certain quests that you only get if you have the profession (as primary or secondary) for that quest. Like, if you are a ranger, some NPC in some town will give you a quest, but if you are not a ranger, nothing happens when you talk to that NPC. I would imagine that if you change your secondary profession to a new one, then these quests will become opened to you. But I am not sure, haven't tried it.

After you change your secondary profession, map to the various towns, for example Yaks, and see if any NPC now suddenly has an ! over their heads.

I guess the only other way to get your new class skills is to buy them at various skill vendors in the outposts and towns. But this costs skill points.

yes certain npc's only trigeer if you ar ethe correct profession. for example there are about 8 mesmer skills in sardelac sanitarium to be had through various missions. these missions only trigger if you are a mesmer

You can get a few skills through quests that are only available to certain primaries. The rest you will have to purchase.