I'm not aware of a built-in way to achieve this - accordingly, depending on the number of questions in your topic(s), it might be easiest to just visit them manually and adjust the topics as required.
However, in case you have too many questions for a manual approach, there is an experimental (!) Confluence Questions REST API available too as of recently:
The following Confluence Questions REST Resources would seemingly allow to automate this:
I haven't tried this myself yet though, and the API definitely seems experimental at this point ...
@Dan Rumney - my answer is complete now. Sorry for the iterative approach, but I encountered the odd restriction that "Your post was rejected because it contains blacklisted words", and the resulting post is now word by word identical to the one that I wanted to submit in the first place, alas ...
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