How to change your online broker

Hi there

Does anybody know how to change ones online broker?

There are e.g. Interactive Broker, DeGiro with a low cost advantage over e.g. Swissquote.
But how to move the portfolio?
Selling - Buying?
Or is there something like a service with lower cost?

Does anybody has a hint on or experience in that topic?

Greetings
Cutter