ratputin on "Change category excerpt style after certain date."

I have a category page that currently displays a thumbnail, title excerpt and read more link for each post that has been entered under it. I was wondering if I could change the way that delivery is presented after a certain period of time. for example:

Posts less than 2 weeks old show:
Thumbnail, Title, Excerpt, Read More

Posts older than 2 weeks show:
Title (either linked or with Read More at the end)

Both need to be on the same page.

Any ideas?
TIA
~Rat