Basically everything that has been set up using the connection will need to be re-done. This includes but may not be limited to:
Synchronized Data Extensions will be removed and need to be configured again
New Synchronized Data Extension will most likely have another postfix (e.g. Salesforce_Contact_1 instead of Salesforce_Contact)
- you need to adapt all the Automations/SQL queries that reference those DEs
- Filtered Data Extensions based on the Synchronized DEs won't work
- Access to Synchronized Data Extensions via AMPscript in Emails and CloudPages needs to be adapted
If you use Salesforce Data Events in Journey Builder, you need to re-create all of them in order for them to be connected to your new org. Changing the Entry event requires re-configuring all the decision splits, Salesforce Activities, Goal/Exit Criteria, etc.