The retroactive aspect is the part that has the best chance of being deemed unconstitional:
US Constitution Establishment of Copyrights and Patents
Article I, Section 8. Powers of Congress
The Congress shall have the power ...
[paragraph 8]
To promote the progress of science and
useful arts, by securing for limited times
to authors and inventors the exclusive right
to their respective writings and discoveries;
You have to squint pretty hard to get retroactive extensions to fit under that language.
-Blake