Becoming another path is the art of sub-path (or class rededication) and can be very complicated. It is used by allowing you to keep the skills and spells you currently have and to start new as a different path. The benefits are obvious: the spells and skills of two paths. However there are special items and spells only available to those of a pure path. Choose wisely before subbing.

Sub from Priest

Rededication Requirements
Base: 6600hp/7250mp
10 Ard Ioc Deums
60 000 000 experience
To sub from priest you will need 60M exp, 6600 base hp, and 7250 base mp. Once this is achieved you will require ten Ard Ioc Deums (dropped in Kas Mines) and then go to the Temple of Veil (choosing).

You may start your sub-path to any class you wish, however the most common priest sub-path would be to wizard. This will make you a "Prizard" (Priest-Wizard): which basically will be a wizard with healing and cradhing abilities. When planning for subbing, you must be very careful with your stats, make sure you will be able to learn all the spells you wish to learn before you reach 99. Council the spell and skill sections for information of stats to learn spells.

When subbing make sure you are wearing the right gear to give you the most advantage in your new path. A dark belt and stone axe are most of the common helpful items to start you off.

Stat Plans
Here is a stat great stat plan for the sub from Priest to Wizard created by MystRoque.
There are excellent Sub stat plans and information in the class special forums at PlanetDA, created by Aes.
Also great stat plan information can be found at DAHelp in the Guide section.