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.
Tracking events won't be pushed to the old SF org afterwards.
If a new connection is made, tracking for previous sends is not pushed to the new org.
Triggered Sends will still be connected to the old org when disconnecting, so you need to deactivate them manually.
The mapped business unit(s) and users need to be remapped
For help on disconnecting the account, also check out the documentation article "Disconnect Account".