depends on how it works, i remembered my ex-company did the same thing before, but under the new contract, they have to put my years of services (back dated from the oringial employment letter) and list out all the benefit and welfare which will be the same as the old one.
and even the contract being signed at the present but i did carry my 8 yrs of services to the new one and if they lay me off, will have to pay me the 遣散費 for those 8 yrs + whatever yrs of services i had under the new contract...
so you could just negotiate with your boss for the terms under the new contract.