I bought a 550D.
Initially I had ONE CD card, but after a while I thought it wise to get more cards.
In the manual it has "instructions" on how the camera deals with when you takeout/put a card. Be it the original or a new one.
My old 300D has this quirk that every time I put back even the same card, the numbering starts something else.
The 550D has an option (quickly looked in the manual's index and can't find it now) but so when you change cards, it changes numbering, but the same numbering for the same card.
Alas I've changed cards, but it is the same series of numbers as the other card.
I shall look though the manual again after posting but if anyone else can clarify to me what I think I read and what really happens.