Create a new field and choose any fieldtype form the list. Save.
Go back in and edit fieldtype.
The drop down list for Type has now been limited to Email Address, Text Input, Rich Text Editor, Text Area and URL.
Note: This thread may be needed for reference.
This is intentional, please see the fieldtype compatibility definitions for addon.setup.php files.
In other words, if you create a field that defines itself as storing data with a given compatibility, ExpressionEngine only offers the ability to change to other fields that will not be a destructive operation.
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