I have found that the new K9 will not attempt to re-send failed Outbox messages. I do not think that your issue is related to the upgrade. The new K9 does the same thing to me when I try to send while out of cell coverage (such as hiking in a National Park) unrelated to the upgrade.
The only way of sending that I have found is to choose “send as a new message” in the message options. This will bring up an edit window with your old text pre-populated. You can send this new message just fine.
The old message will remain in your Outbox until you manually delete it.
This is a kluge workaround, but it at least prevents you from needing to retype your original message(s).