Job change system?

I started a new project recently and I want to create a two systems related to characters' jobs:
1) One of the characters will be able to freely change her job at any time in the game. She starts with only one job, but as time passes she gets new ones and can freely change between them. The only way I managed to create that was by making key items that change her job, though I haven't implemented this system yet. She has a total of 4 jobs.
2) The second is a simpler one. Some characters have two (and only two) jobs and can freely change between them (Smaurai to Archer and vice versa). The last time I tried to create one years ago, I used that "Common Event" tab, but I can't remember HOW I did it.

So, does someone knows how can I make job change systems like that?

You can do it via events, or you can also use Yanfly's class system

Yep, you can do this via event, set the event that change the jods/classes, a choose command event would be useful too.

If you want change the jods/classes at a certain point, create a NPC for that.
if you want change the jods/classes at anytime, create a skill or object with a common event in the database.

Vanfly's is for RPG Maker MV, I'm using VX Ace.

EDIT: I'm trying to implement the system and it is working fine; The only problems I have are:
1) When changing classes, sometimes character's HP and MP go down due to the dfference in stats.
2) I don't know if the system will work with more than 2 classes.

Nope, https://yanflychannel.wordpress.com/rmvxa/core-scripts/class-system/

I was accessing the wrong site, then.
Thanks. This will be very helpful.

With the stat differences, the simplest option would be to have the HP/MP for the two classes a character can have, set to the same. More cumbersome you could have a variable that records the characters MHP before, then after the class change, and then modifies the current MHP parameter by the difference.

For more than two classes, a choice with conditional could be used to track which class is currently selected.

Here's a rough example of both, which could be expanded improved:

I already added the script, but this sounds very helpful as well.