What happens if we decide to change supplier, i.e Active campaign --> MailChimp. How do we recognize/work with that?

Any changes to secondary integrations i.e. non-CMS systems are handled very naturally. If the user makes the jump from one email service to another, the data flow will naturally stop on the old service due to no more data being generated there. After they have attached their new service we will pick up from where new data was generated and continue to pull data from thereon out.