Well, as explained by studiotsunami, you can not change DT's "defined categories" of the pics, which are already online.
however, you can make as many categories, as you choose, by making your own collections.
You can give name of your choice to your collections.
Arun. posted in how to categorize my uploaded photos ??
I think, once the pic is on line, then, you can not change the EXIF data.
You can perhaps disable the online image, and resubmit a fresh image after changing EXIF data ( date in your case).
For changing date, I would usually use corel draw or photoshop as a tool to save a fresh copy of image, and that automatically changes date to current date.
Hope, this helps. posted in How to change the date when photo is taken?