It is known as the Transfer of Undertakings and Protection of Employment. It basically means you transfer your contract of employment to another employer who is legally obliged to honour all your existing conditions eg.. Holiday entitlement, Salary, Pension etc... However, once you transfer, your new employer can then begin to negotiate new conditions for you, but any negotiations MUST go through your union and be agreed by them. Don't be forced into changing any part of your existing contract of employment. If you do feel pressured, make sure you put it in a letter that you are doing it under protest. That way, your new employer cannot say you simply accepted this new role.