I did too and I still haven't done mine because you need to sned it off to NL so they can change it and send it back. Once the evtn season is over then I'll get mine done.
If you go onto the website you can download a form that you send with the passport confirming the change of details. If you PM me your email add I'll forward it over. I still have it on my comp.
I think if you want it done quickly or happen to live in the south west you can get it stamped by the British Warmblood Association (I think thats the name) which is in exeter.
I can second The British Warmblood Society.
I did mine through them and they were extremely helpful and quick, much easier than sending to NL, they will over stamp for you. The link is below.
I sent mine back to kwpn in NL, recoreded delivery and had it back in a few days, you can email them and they will send you all the forms you need to fill out.