Create a new account on bar.social (unless Alice already has one there).
Upload the whole alice@foo.social channel to the new account on bar.social. This can be done either by having bar.social download it from foo.social or by manually downloading the channel from foo.social to a file and then manually uploading this file to bar.social; the latter has been experienced to be more reliable.
Change the identity of the channel which is still alice@foo.social at this point to alice@bar.social.
Have all contacts on servers that understand nomadic identity change their connections with alice@foo.social to alice@bar.social. Hubzilla and (streams) users will only notice the move because the ID has changed, but everything will work the same after the move.
Delete the old instance of the channel on foo.social.
If the account on foo.social has no more channels, delete the account on foo.social.