I recently traded for a player who will be a FA at the end of the season. I was told by his previous owner that he would resign a long term deal (extend contract). Now the trade has been completed and the player is not willing to sign and he is going to test the FA market.
I have made trades similar to this many times and have not had anyone "switch demands". Have I just been lucky?...or did i get bad information?

Could be bad information, but players could also change their minds when going to a new team for a variety of reasons. If your record is worse than the other team's, or if you have bad coaches, I believe those are two reasons a guy could change his mind and walk next year.

In the past, I have traded for a guy who refused to sign an extension with his current team. Because of this, I gave up very litte, based on the thought he'd be a rental for the rest of the year and then I'd lose him. Long story short, He changed his demands when he came to my team and I was able to lock him up long term.

Moral of the story, guys can change their minds on FA.

If he would have re-signed, and you wanted him, why did you not just have the other owner sign him before the deal. This would take away the risk that he might change his mind and not re-sign.

And who knows, you might have gotten snookered by the other owner.