The Rosan Bishop Class Up kit is specifically for the Priest model kits.
While you can fit this onto the Liber version without issues, the colors are meant for the Rosan Priest. Besides the colors, the main difference is a lack of any metallic plastic, and it includes a horned mask with a gem in it, instead of a crown.
Several of the parts are specific to the Priest's armor, though you can use the wrist armor, back plate, knee pads and the front waist piece on most other models without issue, while the staff parts, chest armor and shoulder parts are specific to the Priest's armor.
This Class Up features notably fewer parts than other upgrades (12 vs 17 for the Warrior).
Perhaps a little frustratingly, it functions more as a completion of the Priest and for filling out the five very prominent and empty 3mm hard points on the front, rather than a Job change. And it *still* leaves the feet bare.
That being said, it does flesh out the model and change the silhouette in a pleasing way, giving the more neutral Priest some much needed character.
If you want to go more towards a Western-styled 'cleric' direction, an (admittedly more expensive) alternative is to buy an additional corresponding Knight kit (since all of the colors are a 1:1 match for both factions). This allows you to add the nicely matching Knight armor to the feet, knees, thighs, lower arms and wrists of the Priest, giving it a full plate look.
You can combine the Knight's sword hilt and the head of the Priest's staff to make a flanged mace, which pairs nicely with the kite shield.
You still have the option to add the Bishop parts, if you want to, but this combo of a heavy armed cleric allows you to maintain a clergy appearance, rather than the more royal looking Liber Bishop or the cult caster looking Rosan Bishop.
At the end of the day, these are your kits to modify however you want.