I call it typical human behavior. Human memory is unreliable. It is normal for people’s story to change over time. The only exception is if someone is working from a script and spends a good deal of effort to make certain they have all the details nailed down.
Givens' changing his story is not as fishy as Belin's failure to point it out to him. Belin created a memo regarding Givens' previous statements. He knew darn well that Givens had NEVER before said anything about seeing Oswald on the sixth floor AFTER the elevator race, and that he (and all his co-workers) had previously said they'd seen (or heard) Oswald on the fifth floor as they'd raced the elevators down for lunch.
Strong support for their collaboration in Givens' lie, moreover, can be found in the subsequent statements of Belin. Decades after Sylvia Meagher first pointed out the change in Givens' story, Belin continued to insist Givens was one of the most important witnesses. He was blowing smoke.