The thing is too is that you are planning on giving them such short notice too that they need to pay during your 2 weeks off. They have probably made other arrangements while you are closed and will have to pay for that too. Regardless of whether or not you think they can afford it you really don't know their financial situation. You have had 30 weeks to save up for your time off.

That being said you have every right to make changes to your contract. With that type of change though I think more than 1 month notice should be given. You also need to be prepared for them to go elsewhere if they don't agree to your new terms.