Update documentation to mention using blake2 with "transfer" action.

docs/how-to/upgrade.md

@@ -160,17 +160,22 @@ Then, run the `rcreate` action (formerly `init`) to create that new Borg 2
 repository:
 
 ```bash
-borgmatic rcreate --verbosity 1 --encryption repokey-aes-ocb \
+borgmatic rcreate --verbosity 1 --encryption repokey-blake2-aes-ocb \
     --source-repository original.borg --repository upgraded.borg
 ```
 
-(Note that `repokey-chacha20-poly1305` may be faster than `repokey-aes-ocb` on
-certain platforms like ARM64.)
-
 This creates an empty repository and doesn't actually transfer any data yet.
 The `--source-repository` flag is necessary to reuse key material from your
 Borg 1 repository so that the subsequent data transfer can work.
 
+The `--encryption` value above selects the same chunk ID algorithm (`blake2`)
+used in Borg 1, thereby making deduplication work across transferred archives
+and new archives. Note that `repokey-blake2-chacha20-poly1305` may be faster
+than `repokey-blake2-aes-ocb` on certain platforms like ARM64. Read about
+[Borg encryption
+modes](https://borgbackup.readthedocs.io/en/2.0.0b3/usage/rcreate.html#encryption-mode-tldr)
+for the menu of available encryption modes.
+
 To transfer data from your original Borg 1 repository to your newly created
 Borg 2 repository:
 
@@ -191,7 +196,7 @@ confirmation of success—or tells you if something hasn't been transferred yet.
 Note that by omitting the `--upgrader` flag, you can also do archive transfers
 between Borg 2 repositories without upgrading, even down to individual
 archives. For more on that functionality, see the [Borg transfer
-documentation](https://borgbackup.readthedocs.io/en/2.0.0b1/usage/transfer.html).
+documentation](https://borgbackup.readthedocs.io/en/2.0.0b3/usage/transfer.html).
 
 That's it! Now you can use your new Borg 2 repository as normal with
 borgmatic. If you've got multiple repositories, repeat the above process for