From the UK Act:
Duration of copyright Section 12: Duration of copyright in literary, dramatic, musical or artistic works 12.-(1) The following provisions have effect with respect to the duration of copyright in a literary, dramatic, musical or artistic work. (2) Copyright expires at the end of the period of 70 years from the end of the calendar year in which the author dies, subject as follows. SCHEDULE 1 Copyright: transitional provisions and savings 3. The new copyright provisions apply in relation to things existing at commencement as they apply in relation to things coming into existence after commencement, subject to any express provision to the contrary. 5.-(1) Copyright subsists in an existing work after commencement only if copyright subsisted in it immediately before commencement. Duration of copyright in existing works 12.-(1) The following provisions have effect with respect to the duration of copyright in existing works. The question which provision applies to a work shall be determined by reference to the facts immediately before commencement; and expressions used in this paragraph which were defined for the purposes of the 1956 Act have the same meaning as in that Act. (2) Copyright in the following descriptions of work continues to subsist until the date on which it would have expired under the 1956 Act- (a) literary, dramatic or musical works in relation to which the period of 50 years mentioned in the proviso to section 2(3) of the 1956 Act (duration of copyright in works made available to the public after the death of the author) has begun to run; Unless I'm missing something, reading all these sections together, esp. s. 5(1) of the Schedule, spell non-retroactivity of the stupid, stupid, stupid 20-year extension to life+70. If the life+50 term had already run out, the life+70 term does NOT apply.