Hi, network,
I don't know of any easy way of correcting the creation date on large number of clips. Converting a clip creates a new clip with the creation date of the date of conversion. So there would be no previous creation date to which to revert. Maybe there is some software that can do that, but I am not familiar with any.
If you open iMovie you can select a range of clips in the media browser and then do a Modify/Adjust Clip Date and Time. You will get this box:
The box indicates that you can change the creation date of a range of clips by checking the box "change file creation dates as well". Of course this would only work with a range of clips that you know to have the same creation date, and would require that you know what the creation date is. It wouldn't be so bad with clips that you know bear a certain creation date, like birthday parties, or Christmas Day, or 10th Anniversary, or Superbowl Party, etc. Those could be changed in a batch. It's still a pain, I agree. But at least you might not have to change all of your converted clips one at a time.
-- Rich