RP change due to change in employer

Hi,

I am an Indian citizen, currently working in Belgium on long term Work Permit. Currently my Single Permit (RP) of type "A" is valid till end of 2020.

I had got an offer from another employer in Belgium and then they processed to get my work authorization changed to them. Now that has been approved and I am asked to go to commune for getting my RP changed.

In this situation, if I get my RP changed with the approval documents from new employer, will that mean I will not be able to work for current employer as soon as the RP is changed? I am asking this because I need to serve the notice period for my current employer before I can start working for new employer.

Or can I get my RP process done in the commune and then continue with current employer till the end of my notice period and post that start working for new employer?

Thanks,
RJ