Thanks to those who responded. This is what I discovered/performed.
From the main page https://swi-prolog.discourse.group/ click the browser refresh button.
Click Categories
Click on Uncategorized
Click 
Scroll down so that all topics show in list. Topics not visible can not be selected so they all need to be visible. Hopefully you don’t have thousands.
End of page should show 
At top of page click 
Click 
Notice the number of topics selected, make sure the number is correct.
Click 
Change category
Click 
The progress will be shown.
This now shows the topics listed with the category General
but the General category is not showing the correct number of topics in the category drop down, e.g.
and the category Uncategorized still shows a number, e.g.
and still list those topics, e.g.
Seems patience is a virtual here.
After a few minutes the values are changing, e.g.
In checking the next day all topics have been moved.
Note: There is also a related Discourse setting allow_uncategorized_topics
Allow topics to be created without a category. WARNING: If there are any uncategorized topics, you must recategorize them before turning this off.
When this is enabled new topics created without a category will be assigned the category Uncategorized