As far as I believe, you don't have to retro fit and can just simply change the washer - but if you ever have to change the entire valve then I think you are meant to change it to a part 2. The more I read the page in the regs book, the more I'm convinced they contradict themselves!!!!
Reading it today, as long as there is a dcv immediately upstream of the part one, then a part one can be fitted anywhere, even in a wc. Page 7.4, R16.2 (recommended action by water industry).
But DETR Law states (same page) requirements for wc's is a part 2,3, or 4. Anywhere else part 1,2,3 or 4.
I give up!