That depends on how we define fuselage. Sure you can keep the diameter, but one must not forget that outside the nose section the A330 still differs structurally from the A300, even though it is based on the fuselage of the A300.
You know as well as I do, that aircraft design is not Lego and if you want to do it right you might only end up with the nose (Airbus does have 2 better designs now) and the fuselage barrel plugs as re-useable. The central fuselage needs to change with a new wingbox / MLG and depending on how short you want to go, you might need a bigger tail (again Airbus has 2 better designs in production) as well.
But the biggest part imho, is that you want to use new materials and optimize for modern production.
In what way differs the A330 fuselage?
If you take a cut across it, they are identical. Lengthwise you have the same distance between frames and same height of the frames.. Same windows. The difference is not in design, but that single parts are beefed up, for the greater stress on the A330. Bigger wing box on the A330.
I would say, that there is a bigger difference between a 777 and a 777X fuselage and also between a 737 classic and NG fuselage, than between the A330 and A300 fuselage.
I would see no problem to produce a hypothetical new A330/310 fuselage on the current A330 lines.
It differs by all those parts beefed up for the A330. I really do not see why the engineers at Airbus would save any time by starting with the old design, instead of simply doing a new fuselage with the same diameter (or a double lobe like the A350). Then you can use all new materials and construction methods that have become available in the last 3 decades.
I tried to tell you, that the difference between an A330 fuselage and A330 fuselage that you claim to be, is just not there. The A330 is a stretched A300 fuselage with the needed changes for a big increase in MTOW. There are different parts between a A330-300 and an A330-200 fuselage in certain places. They would add the third.
There is more difference between a 777 and 777X fuselage, still produced on the same line.
What new materials if you build an Aluminium fuselage? Aluminium Lithium does not seem to be the big advantage it was claimed to be, the big disadvantage to Aluminium Lithium alloy is, that it is rather brittle, even though it has a higher strength. I assume fatigue testing showed that you still need a similar weight as in aluminium 7075 in regards to skins.
Going for CFRP fuselage would negate any advantage of building something similar as the A330 on A330 production line, you would need a new dedicated production.
The main point to looking at the old A300/310 fuselage is, that it was a rather light fuselage compared to its size. Both the A300 and the A310 are lighter than the 767-300 and 767-200 respectively. At that time when Airbus developed the A330, customers just wanted to have bigger frames.
I still do not believe in the big NMA market, the A310 and 767-200 had similar low sales numbers. But I believe that the design of the A300 fuselage, is the optimal design for a small wide body, in regards to weight to possible load of freight and passengers. The draw back of the A300 in regards to range would disappear today, more range would come through modern engines, rather than added weight.