This, of course, is directly contradicted by Mr Shelley's same-day affidavit account:
“I ran across the street to a corner of the park. I ran into a girl crying and she said the president had been shot. This girl's name is Gloria Calvery, who is an employee of this same building. I went back to the building and went inside and called my wife and told her what happened.”
Mr Shelley had been best man at Ms Calvery's wedding just a few short months before!
So--------------the question is: Why did Mr Shelley change his story?

I don't think Shelley changed his story. Instead he simply never told the complete story 100% in one interview. It's human nature to be selective in their memory. What a witness states is also influenced by the circumstances he is in when he makes the statement. The quality of the answer is always determined by the quality of the question. When a witness signs an affidavit, he/she is actually signing a synopsis of all the statements/answers he has provided to the person taking the affidavit. The witness does not write the text and although he/she is always asked to agree with the content of the affidavit before signing it, most people simply sign the damned thing to get it over with. That's why lawyers always tell clients to never make any statements to the police.
As for testimony, the witness' answers are predominantly guided by the the questions that are asked. Those questions are most often than not agenda driven prepared to obtain a certain desired result. A witness very seldom will get an opportunity to provide information not asked for.
And then there is the human factor, which clearly shows that witness statements, under oath or not, are frequently (if not usually) not consistent.
The hypocritial LNs always say that you can only believe those parts of witness statements that are corroborated by other evidence. However, when it comes to Victoria Adams' testimony they suddenly rely on it as if it was written in stone, despite the fact that it doesn't match other evidence, when the location of the sighting of Shelley and Lovelady is concerned.