Yes - the project is to provide a third passenger line in each direction between Finsbury Park and Alexandra Palace, and runs for the next two years, til Dec 2013.
The main purpose is to allow Moorgate trains to/from the Hertford loop to run independently of the ECML, various track and crossover rearrangements are also taking place, the main limitation is that intermediate stations are not getting extra platform faces initially...
Don't know about previous platform numbering though...
PS - there is Thameslink platform lengthening to do as well, so I expect more platform work will also be happening, it may make sense for this to be done at the same time as the third up platform...