Doesn't Thunderbird provide any means to do this ?

Unfortunately, Thunderbird’s calendar UI doesn’t currently support bulk moving events between calendars with category assignment in one step.

If you wanted to try a workaround, you could to download the ics file and then edit the entries contained therein to have categories:medical included before re-importing the file to the desired calendar in thunderbird.

I hope this helps!

@Thundamike : Thanks so much for your help.

I am afraid that not all data is preserved during the export-reimport process. (At a minimum time_created and last_modified will be lost. Probably more.) Therefore, I would rather solve this inside Thunderbird, if possible.

Unfortunately, Thunderbird’s calendar UI doesn’t currently support bulk moving events between calendars with category assignment in one step.

If it is not possible "in one step", could it be done in 2 steps one after the other ?

i.e. (1) from [calendar: "medical"] --> [calendar: "home"] (2) from [category: "none"] --> [category: "medical"]

?

This is a very old question with solution but it may help : https://support.mozilla.org/en-US/questions/1241332