ISCHANGED can only be used when the WFR is triggered "Every time the record is created or edited"

ISCHANGED

Description:	Compares the value of a field to the previous value and returns TRUE if the values are different. If the values are the same, this function returns FALSE.
Use:	ISCHANGED(field)and replace field with the name of the field you want to compare.
Validation Rule Example:	The following validation rule prevents users from changing an opportunity name after it has been created:NOT(ISCHANGED(Name)). NOT(AND(ISCHANGED(Priority), ISPICKVAL(Priority, “Low”)))is a validation rule that ensures if a user changes the Priority of a case, the new priority cannot be “Low.” NOT(AND(ISCHANGED(CloseDate), ​OR(MONTH(CloseDate) <> MONTH(TODAY()), ​YEAR(CloseDate) <> YEAR(TODAY())),$Profile.Name <> "Sales Manager"))is a validation rule that prevents a user from changing the Close Date of an opportunity to a date outside of the current month and year unless that user has the “Sales Manager” profile.Note $Profile merge fields are only available in Enterprise, Unlimited, and Developer Editions.
Tips:	This function is available only in: Assignment rules Validation rules Field updates Workflow rules if the trigger type is set to Every time a record is created or edited. Use the NOT function to reverse the return values of TRUE and FALSE. This function returns FALSE when evaluating any field on a newly created record. If a text field was previously blank, this function returns TRUE when it contains any value. For number, percent, or currency fields, this function returns TRUE when: The field was blank and now contains any value The field was zero and now is blank The field was zero and now contains any other value