I've done Job Change as soon as I turned lv. 20 (on my Mage & Fighter). Both easy as hell as long as you have pots, stones, defense/aim scrolls, and vitality/evasion scrolls. That's all there is to it
As for your question, yes it would be waaay easier at a higher level because...
a) You can hit alot more on the monsters in the Quest.
and
b) Your shadow's HP doesn't change depending on your level. This will never change:
- Shadow Fighter: 7980 HP
- Shadow Archer: 5563 HP
- Shadow Cleric: 5260 HP
- Shadow Mage: 4468 HP