i always use LEAVE EXISTING that way it won't mess with your bouquet, but if you find any new channels you will have to add them to your channel lists but that only takes a few seconds rather than the hour or so it can take you to get your bouquet just how you like it again
Also, before you start your scan, use your FTP software (I use FlashFXP) to copy both your services and bouquets files to your pc. That way, if you do freck it up - you can just copy them back from your pc to your Dbox and all is back to normal again.
Easy peasy - Japanesey!
the new channels will be listed in a bouquet named OTHERS, but you also might find you will lose one or 2 that you already had ie i lost itv2 and itn news channel . if this happens you can try scanning again or manually add them to your service file providing you saved your previous one to your pc before you scanned.
I am having the same problem I put the new sporster image on I am misin loads of channels when I do a scan I get les than I already had I then got my mates services file and put that on still no good he has more than me.