How to replace the buttons (attachment) only w/ Slack interactive buttons response
You can only replace the complete message, not just a part.
There are two options to update the original message:
Respond to the Slack request with
{"replace_original": true}Use
chat.update
If your original message was not of type ephemeral you will get a copy of the original message as part of the payload from Slack in the original_message property, which can be helpful to update the exchange the original message.
See this page in the Slack documentation as reference.