This would convert the date field to string if and only if the account type = Prospect or Customer, else it would display the date field in the date format for anything other than Prospect or Customer.
IF(ISBLANK( Fiscal_Year End__c), NULL, IF(OR(ISPICKVAL(Type,"Prospect"),ISPICKVAL(Type,"Customer")), TEXT(DAY(Fiscal_Year End _c )) & " " & CASE( MONTH( Fiscal_Year End _c ), 1, "January", 2, "February", 3, "March", 4, "April", 5, "May", 6, "June", 7, "July", 8, "August", 9, "September", 10, "October", 11, "November", 12, "December", "None" ),TEXT(Fiscal_Year End__c)))
IF(ISBLANK( Fiscal_Year End__c),
NULL,
IF(OR(ISPICKVAL(Type,"Prospect"),ISPICKVAL(Type,"Customer")),
TEXT(DAY(Fiscal_Year End _c )) & " " &
CASE( MONTH( Fiscal_Year End _c ),
1, "January",
2, "February",
3, "March",
4, "April",
5, "May",
6, "June",
7, "July",
8, "August",
9, "September",
10, "October",
11, "November",
12, "December", "None" ),TEXT(Fiscal_Year End__c)))