Presidential candidates have been getting security briefings since Eisenhower: https://www.cia.gov/library/center-for-the-study-of-intellig...

So it's not the case that there was a huge cache of information that he didn't have access too until entering office. It could have been the case that there were some details that weren't made available to him, but that seems unlikely to amount to anything that would lead to a complete reversal of policy.