In the HUNT FOR RED OCTOBER, Alec Baldwin played the role of Jack Ryan. Harrison Ford took over the role of Jack Ryan in CLEAR AND PRESENT DANGER and PATRIOT GAMES.
And Ben Affleck in THE SUM OF ALL FEARS.
The Ryan series is quite full of switch-a-roos. Here are but a few (major recurring characters from the books):
Cathy Ryan: Gates McFadden in HFRO, Anne Archer in PG and CAPD, and she was not present in SOAF.
John Clark: Willem Defoe in CAPD, Liev Schreiber in SOAF.
Judge Moore: Ryan Reinhardt in HFRO, Dean Jones in CAPD.
(not listing Sally Ryan, since it is not uncommon for children to be played by different actors, especially to show time has passed).