Complex sentence: While these carried twice the bombload of the earlier aircraft, and had significantly better performance, it soon became outclassed, although 99 Squadron was forced to retain the Heyford until October 1938, when it converted to Vickers Wellington monoplanes.
Split into simpler sentences: While these carried twice the bombload of the earlier aircraft, and had significantly better performance, they soon became outclassed. However, 99 Squadron was obliged to retain the Heyford until October 1938, when it converted to Vickers Wellington monoplanes.