I've just been through this process and retained the registration number '911 ED'. I found that the bike must have been on the road for 5 years or more, It needs to match the V5C, i.e colour, engine and frame number. I tried to do the transfer on line but my online application was rejected and I had to download the paperwork and send it to DVLA. The bike was inspected by a local firm then the application was approved. A new V5 arrived with an age-related number along with a retention certificate for the plate I wanted to keep.