The difference is the time particle '에' in the noun "예전".
It makes the noun "예전" like an adverb.
e.g.
예전 = 옛날 (both are nouns)
예전에 = 옛날에 (will be followed by "something happened")
아침 = the morning
아침에 = in the morning
\^o^/
2013년 2월 12일
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Simply put, 예전에 is used when describing something that happened sometime ago and 옛날 is for a very long time ago (ie back in the days...).