Does anyone know if you start a level and enter a costume code ex: holocron (for Jedi adventure robes) does that eliminate the holocron that provides that costume, thus causing you to not be able to complete the holocron achievment?

and

If you finish on Apprentice then start over with the same character and swith the level difficulty to whatever the next harder level is, will it still give you the achievment for beating the level on that particular difficulty, or do you have to start all over with a fresh character?

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Well the Jedi Adventure robes is'nt a Holocron you get it at the star of the death star and the best way to be safe with the costume codes is not to enter the ones which give you costumes you can get in game.

if the costume is found via holocron then yes that particular holocron dissapears, but for other costumes such as the Adventure robe and the robes you unlock when completing it, you can only unlock one at a time, and the robe wont save, so either quitting the game or completing the level will automatically disable the robe you unlocked.

the 2nd question im not sure about, i stupidly thought you would have to start again, so i did and lost all my stats! grrrrr. and costumes/ lightsabers.... im wernt happy. lol

Well, I have actually had it not count as me grabbing it. What happens is, say on Kashyyyk, you grab Kento's robes and if you put them on immediately w/o saving, then you lose the unsaved data, which is you grabbing the holocron. It messed me up the first time I played through b/c I always grab all the holocrons and I was missing one in that level for a while until I realized it didn't count as me grabbing it. If you save it after you grab the holocron and then switch the costume on it should be alright though.

On xbox 360 that is

the youngling achievement says complete the jedi temple, where is it?

You have to download the extra level from the x-box live market place.