@devildriven I don't believe you should send what you've written there.

You need to clarify that when the contract was agreed there was no limit on the data.
You secondly need to state what change has been made by them, and when this was brought in.
You need to establish that you were not informed within the limits of the original contract.
You lastly need to combine these three into a summary stating that they have breached the contract.

Have you been charged for using >500MB data yet? If they don't actually change your conditions then they haven't changed the terms without informing you (thus there is no breach).

Stating that they may start to charge you for over 500MB in future would not be classed as an anticipatory breach in my opinion