In 2002 the rules were clear: any change in flights up to and including the first international flight requires recalculation of the fare (and in the case of WAW it is a rather large increase now). I haven't checked whether that rule is still in effect in 2005, but I was burned by it in 2002. Of course the RTW desk has no obligation to warn you that there will be an additional collection with your change; the BA agent at WAW will either accept your coupon or do the add collect. Which do you think BA is doing these days?

I don't know the answer, but I think there is significant risk, including the risk that you cannot reinstate your original booking and the ticket is now unusable (all you can do is refund it or reissue it at the new ex-WAW xONEx fare). I suppose a chat with the agent at WAW to induce them to accept your original flight coupon would be greatly to your advantage. WAW is known to be inducement friendly. Once you have flown the first coupon, no matter what the rules, the rest of the OWE is in the clear.