Al Williamson Secret Agent Corrigan 3 March 1967

I assume that Al was trying to "transition" with this strip and story arc, just because it does channel Lubbers to a degree.  And obviously Williamson at some point would be doing "his" Corrigan 100%, so this couldn't have been intended as a permanent thing.  Gorgeous strip!

I think you (and Jeff) are correct when you note that Al was trying to stay in Lubber's style for at least the first story arc... and that may have been a King Features directive.  When someone has drawn a strip for years, the syndicate probably did not want a radical change in style that scared away the readers.  They tried to mimic the way it had gone before.  Certainly, John Prentice tried to mimic Raymond when he took over Rip Kirby, and John Cullen Murphy stayed as close as he could to Foster when he took over Prince Valiant.  Even Dan Barry mimiced Wilson McCoy's style on the Phantom before all of them slowly established their own styles.