You can't eBGP route private addresses. (well, you can, but if you do, you should be shot). The new ISP would need it's own space to announce (it's not much

You can't eBGP route private addresses. (well, you can, but if you do, you should be shot). The new ISP would need it's own space to announce (it's not much of an ISP if it's got no customers to give addresses too, so it needs addresses. It's also not much of an ISP if it's only got one upstream, so it'll need PI (provider independent) space, allocated to it from it's RIR (ARIN, RIPE, etc). And that'll all be gone soon :)