We document that exceptions should be defined in their own package and add a compiler warning.
If you want to “upgrade” an exception, you create a new exception type. The only thing to be aware of is that the old code won’t be able to catch it.

Discussed in call with Paul and Samuel:

First, reproduce issue

• interface exercise -> produces exceptions -> upgraded exception is different, is not caught by existing case statement, falls through to wildcard

For 2.x

• exceptions should be put into the same package as interfaces
• Update docs to explain
• exceptions needing to be in their own package
• What goes wrong if you don’t do it

For 3.x

• Make catch primitive upgrade/downgrade exceptions
• In the event of a failed downgrade, abort the transaction
• Implement upgrade/downgrade logic within toAnyException
• Feasible, but may end up thorny, would prefer 3.x

Docs: digital-asset/docs.daml.com#804