One way to do it would be to remove the purchase date of the first profile.  Then go to DVD -> Copy.  Then flag all the rest, go to Collection -> Flagged -> Paste, then paste the purchase date.

I've not tested this, but as long as the program doesn't object to pasting a blank date, it should work.

I was also going to suggest the Bulk Edit plugin. I have used it w/o any problems, but as noted (even by the author), always back up your DB