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 ...
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.