Well they did let you change your availability so the first mistake is theirs for giving you Tuesday. Does the rota clearly state the relevant day of each shift? If not, then it is completely unfair, but if the day was made clear on the rota, then they are within their rights to take disciplinary action if the company procedures and management support that decision. And if you've had any prior disciplinary incidents or problems then you probably can't do anything to stop the process, just hope it's not a sackable offence and try not to be confrontational. But what I think I would do, I'd try to talk privately beforehand to someone involved, a manager or someone in the HR dept (if applicable) that you get on with or that you think would be sympathetic to you - if you have a quiet word with someone like that and explain your side, giving a formal apology, that may calm things down and avert the action! Whatever you do though, stay calm and keep the head cool, don't get their backs up even more! Good luck!