I have seen this a few times so I am going to give an example of how to fix it using SQL.
You need to drop the constraints on the fields if there are any then change the column to datetime2.
Then you just add the constraints back.
Remember to back the db up before doing something like this.
Here is an example of how to change the FormInserted and FormUpdated fields in a form.
ALTER TABLE [<Table_Name>]
DROP
CONSTRAINT [DEFAULT_Form_<FormCodeName>_FormInserted],
CONSTRAINT [DEFAULT_Form_<FormCodeName>_FormUpdated]
ALTER TABLE [<Table_Name>]
ALTER column FormInserted datetime2
ALTER TABLE [<Table_Name>]
ALTER column FormUpdated datetime2
ALTER TABLE [<Table_Name>]
ADD
CONSTRAINT [DEFAULT_Form_<FormCodeName>_FormInserted] DEFAULT ('1/1/0001 12:00:00 AM') FOR [FormInserted],
CONSTRAINT [DEFAULT_Form_<FormCodeName>_FormUpdated] DEFAULT ('1/1/0001 12:00:00 AM') FOR [FormUpdated]