Stop JIRA and back it up (as any change to a database can render it unusable, I always urge caution)
Go into the database and find the customfield table. In there, you'll find all the custom fields, so find the one you want and look for its type entry - this will have a class name in it, that includes "datetime". Find a date-only field and check what the class for that is (it will be similar and obvious)
Update the classname for the target field to be the same as the plain date picker.
Now, the thing I don't know is the effect on the data. JIRA stores both types of field as a date and time stamp. For date only fields, the time part is always 00:00:00.000. I do not know if you need to update customfieldvalue to strip out all the times currently stored on each existing field.
When you've finished with the SQL, restart JIRA and immediately run a full re-index, locking the system when you do it.
I have multiple projects that use variations of the same base workflow. The variations depend on the requirements of the project or issue type. The variations mostly come in the form of new statuses ...
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