1 Create a new worklog type
2 Create a worklog entry having this worklog type
3 Go to "Delete worklog type"

You will be asked whether you want to map existing worklog entries to a different worklog type.

a) It always says: "There are currently 0 matching worklogs, that must be changed to another worklog type." Same for all other worklog types. There ARE worklogs.
b) If you delete it anyway, the worklog type is completely removed from worklog entry. There is no worklog type assigned now.