Convert date data without editing all files

I have all my mp3 titles edited with MP3Tag editor, but when I put in the date I entered it as 070421 for July 4, 2021 and I would like to change it to 20210704 since the dates in the format I have always used doesn't sort properly. Is there any way to strip the last 2 digits from the date and then add the 2021 year to the front of the date? I really don't want to have to go in and individually change all almost 60,000 titles.

You could use an action of the type "Replace with regular expression":

Search string: (....)(..)
Replace string: 20$2$1

Are you sure that you do not have any files dating back to the 19-hundreds?

I have every Top 40 song since "Rock Around The Clock" so YES
I understand I would have to change the numbers.

How do I just strip the year from my date column ie: change 070421 to 0704

I have another column with the year in it anyway so I don't need the last two digits for the year.

To have a column with a special content and to modify tag-content are different things. You can define the view of your columns yourself and you can even define in the column-view that only 4 digits are displaid.
So I would not change a regular content of a tag-field just to see it in a special view in the columns but modify the column-view.
View->Customize columns

What do you call a "date-column"? Which tag-field is it?

Replace string: $1