We sent back around £45-worth at the start of the scheme, a mixture of values, and got the replacements back inside a fortnight.We had loads of old 1p, 2p, 5p and upwards stamps at work that I sent back. Expected to get a batch of equivalent value of 1st or 2nd class stamps, we just got new versions of the 1p, 2p, etc stamps.
We included some 1p, 2p 5p etc, and got bar-coded replacements for those, but also some 4p, 8p etc for which there isn't a bar-coded equivalent, so they converted those into 1st and 2nd bar-coded ones, giving me slightly over-the-odds in value.
We kept back some "old" 1sts and 2nds for use during the year and for our Christmas cards; if there's any left over we'll probably swap out before the deadline.
