"designed and certified in the 1960s"...
I think the 744 was certified as a new type (not a variant) due to the completely new wing, avionics, etc. and incorporating the the s.41 lessons, etc. whilst the 744ER was a variant of the 744.
If that's the case, then surely the 748, with new wing, engines fuselage stretch plugs, new materials and structures in critical places, etc. will also have to be certified as a new type?
My point being that whilst the overall shape might be retained (a deliberate decision, Boeing having done consumer studies to show that it is a design which is recognised and trusted by the consuming public, hence incorporates a large element of goodwill), the actual stuff under the skin will be pretty well all new.
Given that it will be first-flying in the 21st century, I'd suggest it will have to meet 21st century requirements for a new type in areas such as "... crashworthiness requirements, damage tolerant structure, larger rapid decompression hole size, different uncontained engine failure rules etc."
How many of you own a Porsche 911? Get online and Google-up a 911 family tree. You can trace the shape back to the 956 but no-one in their right mind would say they're the same underneath. But the punters recognise the shape as a Porsche so that's what the manufacturers have stuck with. The 911 of 2006 has a similar look (a few aerodynamic changes, etc. - rather like a new wing!) to that of 1969 but they're different beasts. Same with the 747 family.