TUPE
TUPE protects your terms and conditions indefinitely. An employment contract is like any other contract in that both parties have to agree to change it.
If you are transferred on a higher salary there is no obligation on the new employer to offer any change such as pay rises to the staff that were transferred from the old company. In essence you stay as you are frozen in time waiting for your salary to erode to the level of the new company.
Any promotion or change of job will undoubtedly be offered on the new contract and not the old contract. Usually over a period of time the number of staff members on the old contract is whittled down to a level where there is a group of a size that the “new” employer will open talks with them or there representatives to change them over to the new contracts. Usually there is a one off incentive offered to make the change but the staff can still refuse to change if they want.