Hey Samra,
Jack here. Damn that's a pretty lame way to run a business. I've had this kind of experience before as I'm also very professional. When I'm at work, I do everything but stand on my head for good tips and just keep the flow going. I know I do a damn good job at it too. I've seen how the number's check out at closing, I'm not stupid. I know a great part of those receipts are due to my hard work and I do take pride in that.
There is a chic at work that busts her ass, came in fresh from Bartending School. She was picking up a lot of shifts for everyone, especially during the summer, and she was damn happy. Suddenly (like you) past employee's are wanting their original shifts back and she lost it - was so hurt, like you.
She was ready to quit - you know, first reaction, shock, hurt, anger, sadness etc. I'd seen this shit go on for awhile and its usually the girls that get shit like that. People take for granted anything they had and always expect it to be there, while someone else is kicking ass and not only serving the customer's, but doing everything they can to keep the flow.
I talked to her later, after she had some time to calm down and pull herself together. Told her that this kind of shit happened all the time and it was f''g ridiculous. I'd noticed how hard she worked, kept a smile even when some of the guys act like total tools and you're dead on your feet etc.
She went in and talked to the power's that be and explained that she felt it was pretty unfair to go from the good shifts (which she had earned) back to the sucky shifts because these other employee's thought they simply deserved it. The owner of the bar has had to scramble to find other bartender's from elsewhere to cover shifts and it can't be a picnic for them to make these kinds of calls. But that's still no excuse.
Long story short, they were impressed that this she went in and stated her case, the unfairness and that she did plan to work at another bar (she'd already had offer's but was being loyal - so much for that) and she'd been promised an increase in wages. She in a very polite way, backed them into a corner, and they kept her and made the old employee come back and work his way back up again.
Its worth a try, you need a job but then again, maybe this job and all the good tips you've described and are using to buy more dope could be running interference in having you change jobs and not have that $ buring a whole in your pocket. And I would ask how you were guaranteed a number of hours, the holidays are coming and you were counting on her word to you.
I don't know Samra..... All I know is I've been where you are and it sucks. How were they going to let you know? After you arrived for your regular shift? That's pretty unprofessional to say the least. Think about it before you do anything and remember, there's always a reason, even if we don't quite see it.
Hang in there - Jack