Complex sentence: As it became apparent in 1939 that this conversion would not be able in 1941/42 when it was originally scheduled for, three of the planned Kreuzer P should be modified to get 38 cm turrets which were already in production.
Split into simpler sentences: As it became apparent in 1939 that this conversion would not be feasible in 1941--42 when it was originally scheduled for. Therefore, three of the planned P class ships were modified to receive the 38 cm turrets which were already in production.