There is a section in B16.5 that gives defined rules on drilling out a blind flange to accept a smaller threaded or socket weld pipe attachment.
I don't have it handy to look up the paragraph, but I remember it has a table with various sizes of blind and attaching pipe, including the size where you have to go to a hubbed 'reducing flange'.
I assume if its covered in 16.5 its still a listed item and OK by 31.3.
