Well, it all depends on a few factors.
1) How much does the first place pay and hw much does the second? If we're talking a couple hundred bucks difference it's not worth bringing up.
2) What is your relationship with the first guy? Has he been loyal to you for a long time?
3) Do you guys play for a living or are you weekend warriors?
You see, everyone's knee will jerk into "Don't be a douche" mode without knowing all the facts. I have long-standing relationships with lots of club owners. I can think or several I know I could call and reschedule with. And I could even tell them why. They get it and wold be totally fine with it. And they'll hug my neck next time I pull up to load out. They use all kinds of bands and it would be nothing for them to call some hometown guys down the street and plug them in the spot. Heck, I have even called other bands before hand and then called the club owner and said.....
"Hey, Mike. We would love to go play a swanky place in Myrtle Beach on th 27th. The Hoss Kats can do your place that night. Is that OK with you?"
(On that occasion Mike said sure as long as I bring him a Margaritaville T-shirt.... which I happily did.)
So there aren't any absolutes here. Most guys here take bar gigs WAAAAAAAAAY too seriously. We're playing rock n roll and selling beer. We're not saving the world.
If you know the guy well, call him and ask. Just be honest. And if he holds you to the date show up with a smile and play a great show.