Best practice for changing an existing Doctype field type

I have a need to change the existing fields in Task labeled expected start date and expected end date. I need them to be date time fields. The simply aren’t granular enough for my use.
I have customized the fields by changing them in the task doctype but I know they will be changed back at the next update.
If I try to change them at the form level I get the warning
“Fieldtype cannot be changed from Date to Datetime”

Dear @MarZah changing an existing core doctype must be by custom app.

How exactly do I change the field “expected_end_date” in Task from a Date to a Date Time using a custom app.

Refer to this link for a complete guide on how to store your changes in a custom app

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Thank you for that but I don’t see how this relates to my question.
Here is the problem I am facing.
A Doctype in ErpNext Named TASK
Has a field named “expected_start_date”
This fields type is “Date”
I need it to be of type “Date Time”
Frappe will not allow me to change the fields type.

These are standard fields and Frappe would not allow you to change the field type. The workaround will be to create your DateTime field and hide expected_start_date field.

How to get the Calendar Task view to reflect these custom fields with start and end hours?