Just tell them. If they are proper friends, they might grumble but they'll understand.

I would go and explain the situation to the new manager - take the hotel booking as proof. Explain that it was arranged before the new system.

If you're worried about getting tongue tied or brow beaten  have it in writing (not a lengthy note) just bullet points. Firmly say that you are sorry but cannot do it as it has all been arranged before the new management rules.

Surely they can be flexible - especially if you're pleasant about it?

Good Luck 

I think it unfair to change the original agreement as a result of a 'new rule'. I would go on the assumption that it still stands as it was arranged before the new ban was put in place (you said someone wrote in the diary?) And the shift swap ban meant no more asking to swap from that day on.

You've already sorted it out and made arrangements that have cost you money, maybe mention that you will need that money back if forced to work?

Good advice already given here. good luck!

I'd go with this chicken and as Soozy said, show them the hotel booking as proof.  I really don't think they can impose a rule change and not honour what has been agreed up to that date 

How long have you worked for them?

Thanks for all the advice.

The thing is the hotel booking was made after I had told my friends about the new system and the issues I was having with cover. So even though I told one of them that I'm having issues with cover. She even said today that she didnt think to tell him off my work problem. That did get to me. 

I don't know how its going to work out , I just know ill get stick from either work or my friends

Chicken, can you not point out that the change of shift had already been agreed with the previous manager ( is that manager still in the store, to confirm they OK'd the shift swap?). Tell them you wouldn't have gone ahead and booked the hotel, had it not already been agreed and marked up in the diary. Stick to your guns and ask them nicely to honour the agreed shift swap - and if all else fails, throw a sickie

I agree with what Yogi and Sweets said, you booked the time off with the previous manager and so it should be honoured - stick to your guns 

And I second that Chicken

But you've already told them that you didn't know if you could go or not haven't you  Chicken, I may be way off here, but, having read other stuff that you've posted, I wonder if the real issue is that you're really not sure if you want to go and could be secretly thankful that the new manager has given you a 'get out of jail free' card?

If that's not the case then just speak with the new manager about your pre-agreed arrangement that needs to be honoured: simples!