It is also wrong to say it's flown for millions of hours on the same design until 18 months ago. Were that the case you would find the fix to be simple.
I think it is. They have to revert to the old welding system that has proved successful over millions of hours. This means that they have to wait until the welds have aged before being put into service; about three months, plus any certification.
All the bulletens are doing is trying to salvage some essential work out of the aircraft before that happens.