AFAIK you don't *have* to get them chipped if you are already passported. If you are applying for a passport now (from June 2009 I think) then you can't get a passport without having a microchip done.
There is only 1 of mine that is not chipped - I had the others done years ago, and he was the late comer!
Only foals and horses without existing passports need to be microchipped, although you can elect to have it done and then have the existing passports updated with the microchip number. It's not compulsary though.