Changing secondary profession

It was a very long time I played GW, only when Prophecies was the only campaign around, and I remember changing your secondary profession was quite a tedious task.

I have now bought all the campaigns including EoTN and I wonder, how is it done now?

You can simply pay an NPC 500g in the following locations, after you have ascended: Senji's Corner [ Factions ], Great Temple of Balth [ Battle Isles ], and Command Center [ Nightfall ]

Or do Quests in Prophecies

Ah great, thank you. And as I've understood, once you've "bought" a secondary profession you can change to and from it to any other bought profession at will? So at the end, if you have paid 500g for all the secondary professions you can change between them all?

Sounds to good to be true :P. But then again I still got my mentality back at Guild Wars 2005 .

Yes that sounds accurate - regarding the "buy it once, use it whenever" method.

You can go to the Great Temple of Balthazar, pay a simple 500g to change your secondary profession.

You have to be ascended of course . Here are the places: The Four Desert Missions (Dunes of Despair, Thristy River, Elona Reach, and Augury Rock), Senji's Corner, and the quest "The Hunted".

Once you've gotten the secondary profession you want, open up your skills inventory (default K) and switch from there.

Wrong. In Factions, you ascend after completing Nahpui Quarter mission (second mainland mission). The quest name is "Hunted!", is in Nightfall (one of the quests soon after arriving in Kourna), and only Elonian characters are ascended after completing it. In Senji's Corner, Command Post and Great Temple of Balthazar you can change your profession at the Profession Changer. Ascending in Prophecies takes more time but is more rewarding, as completing Augury Rock mission will give you 50k experience as long you weren't ascended before doing it, and you can unlock professions with the quests given in the non-mission outposts in the desert (some random phantoms). These quests will only unlock Prophecies campaign professions (Core professions), Senji will unlock only Factions professions (Core, Assassin and Ritualist) and Zuwarah will only unlock Core + NF professions. The guy at Balthazar's Temple has all professions. Oh and in Factions ascending is referred as Weh No Su. The profession changer charges 500 gold for each profession you want to unlock (although that may be obvious information anyways).

When you complete Naphui Quarter you are sent to Senji's Corner, and the actual "Profession Change" NPC's do offer to change to ALL professions, however the quests you get in Prophecies are just that, only Prophecies professions.

IF you have also NF it will allow you to change to /P or /D. Yes I do realize the OP has all campaigns, just posting accurate information, and I do realize you're sent to Senji's Corner after Nahpui Quarter. However you don't go to Dunes/Thirsty/Elona Reach in Prophecies to change your secondary, you must travel on your own (although everyone usually pays a runner or sets a running build) to the outposts (Destiny's Gorge, Seeker's Passage and Heroes' Audience).