Byrna: Sorry about the misuse of the term "upgrade". I meant the A320 or even the larger A321 more as an upgrade of newer design, not necessarily in the same class.
There's a very good saying involving holes and digging.
You suggested that these airlines were compromising safety by keeping their older and less safe A300/A310s instead of changing to newer and safer A320 family aircraft.
A quick check on the Airbus website shows the following soundbite figures:-
A300: up to 298 seats, range 4,150 nm/7,700 km with full passenger payload
A310: up to 247 seats, range 5,200 nm/9,600 km with full passenger payload
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A320: up to 164 seats, range 3,000 nm/5,550 km with full passenger payload
A321: up to 199 seats, range 3,000 nm/5,550 km with full passenger payload
Quite a difference, isn't it? Might explain why these trans-Atlantic operators aren't exactly falling over themselves to do what you suggest.