The water has been spilled!
Man tanks the level loss, even though it seemed only labour intensive & an actual immediate power increase. The description (you cannot obtain bonus stats up to 443) and then subsequent status reveal made it obvious his stats from his prior level remained. He instead acquired (assuming legendary class due to grade of shield) new class skills and now only needs to put in time to regain his level.
If MC is the guideline, leveling will be easy with new job. The game seems to have none to little restrictions on experience gained from drastic level differences. So, dude retains stats of lvl 450 + legendary job = ez kill level 500 monsters as a lvl 1. Power level in a week?