Hi auntiebertie,
I recently renewed my step-daughter's passport. She's now 22 but she got her last passport when she was 11 and it ran out in 2003. I was worried about how difficult it was all going to be as her appearance has changed significantly and the PO clerk said she doesn't look the same person as the photos. I rang the Passport Office and explained the situation and they were very helpful. The man said we just needed to fill out the form as a renewal - not as a new passport - as they know people change from the age of 11 and they use common sense. We just ticked 'Renewal' and paid the extra �5 for the PO's checking service. The new passport arrived in the post just six days later - it was as simple as that.
If you have any concerns then have a chat with the Passport Office. However, I would suggest that you just apply for a renewal and send the old passport off. If it's refused you get your money back so you have nothing to lose. It's also a good idea to pay for the Post Office's checking service. Good luck.