When the services of the employees are taken over by another company, then they have to resign from the current company and join fresh in the new company. When the process is done, a written assurance is given by the new company, that the continuity of service, will be given for, Gratuity, Promotions, etc... Also, their leaves and other balances need to be transferred so that the same can be enjoyed by them. Ensure that the net salary is not being less than the current.
This will ensure, that the employees are not losing on any count, just because of a buy out or carve out.
Regards,