The northbound OHLE between City Thameslink and Farringdon is not energised, it is earthed temporarily (alongside Smithfield sidings) to allow for a temporary fix for a clearance problem with Southeastern 465s and 375s entering Smithfield sidings.
Network Rail said:
Following the reinstatement of the bay platforms (an equivalent turnback facility) at
Blackfriars in March 2012, works will be undertaken to remove the permanent earth,
the in-line section insulator and adjust the OHLE to its design height, thus allowing
north bound services to undertake the DC to AC change over at City Thameslink.
This will need to be covered by a further Network Change Notification at that time.
It should be noted that it is not intended that Smithfield sidings should close. They will
remain as a bolt hole for use by Class 319s and 377 Electrostars – given that these
two types of rolling stock do not have the clearance problems suffered by class 465
Networkers and 375 Electrostars. The sidings will not be usable by the proposed
eventual fleet of 12 –car fixed formation trains.
So the operating procedure when everthing is up and running is that
southbound changeovers will normally happen at Farringdon, with failures running to City Thameslink to tip out passengers (bigger platform?) and returning north on AC via the trailing crossover just south of Farringdon.
Northbound changeovers will normally take place at City Thameslink, 12 car failures will then run to Farringdon on DC and return over the same trailing crossover as mentioned earlier.
Northbound 8 car trains will still be able to use Smithfield sidings as a bolthole if it is decided to just get them out of the way, and return them south later - I suspect that when the 24 tph is running this would be the normal case...
PS - just saw the OP's question about changeover on the move, and this was a definite requirement of the Thameslink stock ITT technical spec.