When you'll have gained enough experience, you'll be permanently changed to a new form, with its own mesh and set of characteristics.
The upgraded form will be available in the next base sequence. Campaign mode only (obviously).
You have a character named Egg-Girl who you wish to transform into the Stupendous She-Chicken after she has gained enough experience.
Egg-Girl's starting value (the one at which you can buy him) is 1280 pts and her maximum possible value after gaining experience is 3450 pts (in both cases, without any Upgradable attribute). Chicken-Girl's starting value is 5160 pts. Since 5160 - 3450 = 1710, a fully maxed Egg-Girl would need an Upgradable attribute worth at least 1710 pts. As there is no Upgradable grade of that exact value, we'll take the lowest superior grade, which is Upgradable (grade 3), worth 3000 pts.
So at the point where Egg-Girl's value reaches 3160 pts or more (5160 - 2000), if she buys Upgradable (grade 3) and goes on mission, at the base screen after that, a new hero will be added to your lineup: She-Ch wait: that's not She-Chicken, but Microwave!?!
Which is normal, since you didn't customize the attribute and Microwave (MP) is the default character.
Built-in character (created/imported in FFEdit): In FFEdit, make sure Upgradable is the last attribute. You also need to set the number of starting attributes to a number lower than the total number of attributes of your character, so Upgradable is greyed out and needs to be bought. After saving your changes, launch the FFX Control Center and select the template you wish your character to change into (in this case, She-Chicken).
Custom character (not created/imported in FFEdit), planned method: Create your character, with any Upgradable attribute. Don't recruit him yet! Exit Freedom Force. Launch the FFX Control Center and select the template you wish your character to change into (in this case, Chicken-Man). Save and close the FFX Control Center. Buying the correct grade of Upgradable attribute in the course of the campaign will now transform your character into the right template (yes, She-Chicken, that's you!)
Custom character (not created/imported in FFEdit), as you go method: Create your character, without an Upgradable attribute. Don't recruit him yet! Exit Freedom Force. Launch the FFX Control Center, save and close. During the course of the campaign, buy your character the correct grade of Upgradable attribute, save and close this character profile. Launch the FFX Control Center and select the template you wish your character to change into (in this case, She-Chicken). Close the FFX Control Center. Your character will now transform into your selected template in due time (yep, the Chicken chick.)
Note: Originally introduced in FFX 2.6.