For EasyGPG we proposed to extend Enigmail Filters to cater to some additional use cases.

This means

1. Adding an "Encrypt To" Filter option to Enigmail that encrypts mails that match the filter criteria
2. Ensure that decrypt works on PGP/Inline mails and extend Enigmail to use decrypt --nowrap for this to leave the signature of combined messages intact.
3. Ensure that a filter that first decrypts and then encrypts to possibly a different key works.
4. When Switching the Encryption key for an Identity offer to Decrypt and then Encrypt mails of this account to the new key.

Optionally

1. Use a Key selection dialog for the Encrypt To filter.
2. Ensure that it is possible to only match encrypted mails with filters.

To be split into subtasks :-)