I know that a modification was made in 1973 regarding cranks/pushrods/pistons

That happened in '74.

1) did the block itself have to be modified at that moment to accomodate these new components?

No.

2) was tere another change in 1975 YM when the B20 went under the 240's bonnets?

Maybe.

3) Can a later (1973-7?) crank (similar to the B21/23 cranks) be installed in an earlier block (1969-73?) with the right bearing set?

Yes. In fact, the main bearings are the same in B20's, B21's & B23's.

But why would you? The late B20, B21/B23 conrods are both heavier & weaker. Volvo made the rods larger to compensate for the cheaper material they used, not because the earlier rods were weak. They then made a major blunder with the rod bolts they used in them, the big counter bore they put in the rod to suit the stupid looking rod bolt introduces the perfect place for them to crack.

4) if 3) is a yes, can an earlier block be bored to 2.3 liters to take advantage of the B23 crank (you probably know why I'm asking :-)

The B23 has the same stroke & you can't bore them that far. I think the late B20 block might have thicker bore to let it be bored to 93mm, but they also tend to be softer.

Volvo Penta has a 2.5 litre DOHC 16V engine with a stroker crank in it, but that crank only fits the later B230 car engines.