I need your help for some clarification about the tax system.

In March 2015, I was initially hired by company A as a consultant. Then I filed for a tax certificate for my consultancy in the RDO of my local area of residence and there were no issues. Eventually the company hired me as an regular employee in August 2016. I left the company in August 2017. I was not advised that I should change my RDO. Now I am joining a company B and I was advised that I have to move my RDO from company A’s area of RDO to company B’s area of residence.

1. What is the process I have to go move from freelancer to regular employee.
2. Do i still need to transfer my RDO to company A’s RDO, or can I go direct to company B’s RDO?
3. Will I encounter any tax issues with this item?