Let's define some variable...
table is the name of you table in MySQL
field1 is you current date field (dd-mm)
field2 is the new date field (yyyy-mm-dd)
First thing to do is add field2...
ALTER TABLE `[I][COLOR="Red"]table[/COLOR][/I]` ADD `[I][COLOR="Red"]field2[/COLOR][/I]` DATE NULL ;
(I used "NULL", but I suggest you use "NOT NULL", this way, you'll have to define a date when inserting new values)
And then, we'll cut field1 in some parts and store them in field2... (here I assume you stored all the dates using dd-mm and not d-m (so 01-01, 02-02, ... shortly, with a zero in the number is lower than 10)
UPDATE `[I][COLOR="Red"]table[/COLOR][/I]` SET [I][COLOR="Red"]field2[/COLOR][/I] = CONCAT('2000-', SUBSTRING([I][COLOR="Red"]field1[/COLOR][/I], 4, 2), '-', SUBSTRING([I][COLOR="Red"]field1[/COLOR][/I], 1, 2))
And if you stored dates using d-m, you can use this :
UPDATE `[I][COLOR="Red"]table[/COLOR][/I]` SET [I][COLOR="Red"]field2[/COLOR][/I] = CONCAT('2000-', SUBSTRING([I][COLOR="Red"]field1[/COLOR][/I], (LOCATE('-', [I][COLOR="Red"]field1[/COLOR][/I]) + 1)), '-', SUBSTRING([I][COLOR="Red"]field1[/COLOR][/I], 1, (LOCATE('-', [I][COLOR="Red"]field1[/COLOR][/I]) - 1)))
And there you go for the transformation 😉