The way I read this is (example dates)

1 Jan: booking made
10 May pm: booking changed for £35
11 May am: wants to change booking again without paying an extra £35

If this is the case then there is a new policy allowing you to change it without paying the extra change fee, as long as you are not changing back to the original flight nor if you wanted to cancel at this stage (you would still pay the £35).