Changing jobs. What’s the procedure for work permit?

Hello everyone,
I recently received a job offer, and the new employer asked me what the work permit procedure would be in my case.
I’m currently employed in Belgium, and my current employer sponsors my work permit. With the new offer, there would be two changes:
Change of employer

Change of region from Flanders to Brussels

Does this require a completely new work permit/single permit application, or is there a transfer/change procedure?
How long does this usually take, and what documents would typically be required from the employer and from me?
Any advice or experience with this would be appreciated. Thanks!