You create separate webaccess and frameworks with their own separate application pools for performance and traceabilty.
Each application pool has its own connection to the database and its own resources to do the work required to process the requests that come to it.
That's the short answer!
To add to Elizabeths comments, the reason it's asking you to migrate is because in previous versions BridgeIT used the framework embedded in Web Access for all it's database access and processing needs. The reason it asks if you want to use your existing framework or create a new one is because of the logon policy you are using. If, for example you have previously logged in to the BridgeIT app using explict credentials (ie the user name and password defined in Service Desk) but your console users connected to your framework use integrated security (eg AD authentication) then you would no longer be able to login to BridgeIT because the framework authentication mode would be wrong, in this case you'd need a separate, explicit authentication framework.
It's really easy to check that your authentication methods are correct - simply click on the 'Edit' button of any application in Configuration Centre, it will tell you which policy it is using. You will need to make sure that you have a framework that has the same policy as your BridgeIT application and that this BridgeIT application is referencing this framework (also visible on the edit button of the BridgeIT application).
Hope that helps,
That clears it up. Thank you both!