Job

Bit of a dilemma here. The company where I am working is planning to sell some european businesses to a potential buyer. We are still in the very early stage but a question came up to my mind.
Is the potential buyer going to recognize the seniority of the employees in case it takes over the business or we will be forced to sign a new work contract? I have been in this company for over a decade and with the Claeys formula it could make a different of several thousand euro in redundancy package (in case they would close down the business few months later). I’d prefer to keep the job but I wouldn’t like to be screwed after.
I am not sure where to gather information about Belgian laws or if I should just join a union.

Anyone has any experience on this?