Additional features, settings, or issues not covered by specific Microsoft Teams categories
Hello Vanessa,
According to my knowledge, the option to change Standard/Private channels is not available for the existing channels. You need to re-create new Standard/Private channels as a workaround.
Regarding to this feature, it is highly recommended to create a new vote in the Microsoft Teams UserVoice forum for feedback. Microsoft always pays high values on customer’s feedback and will develop new features based on the most voted ideas.
Your understanding and cooperation would be highly appreciated.
Regards,
Rick