Ceiling is irrelevant to flying the approach. You might not see anything at DA if the cloud is low but nothing precludes you from continuing to DA and having a look.
Where's your quote from? It doesn't really make sense to me as I only have experience of EU-ops. First of all, for cat I you will have a Decision altitude, not a Decision Height. Secondly, your quote says you can continue below this 'DH' if the visibility is greater than 800m regardless of visual reference. That seems a bit silly.
If you have a vis of 700m you can convert it to RVR only if the resulting RVR after the conversion is at least 800m. This would only be the case if you had HI approach and runway lights, day or night, or any other lighting and it was night only.