Need to convert Single choice field into cascading

[ManuMR]

Need to convert Single choice field into cascading(Standard Field "Activity type" field) 

[Nic Brough -Adaptavist-]

Fields cannot be converted between types in Cloud.

You will have to set up a new field, configure it, copy data you want to keep from the old field into it, and then delete the old one.

(here's a risky database hack for Server/DC that can do some conversions, like single to multi-select, but not for Cloud)