The Thameslink Programme has three elements: Reconfiguring track and junctions so that more routes and more trains can run through the central section; Extending platforms at local stations so that 12 car trains can run; and completely rebuilding Farringdon, Blackfriars and London Bridge stations for both of the previous reasons.
Major work work doesn't start until next March, but preparatory works ("Key Output 0") are happening now. The most significant of these is the replacement of the interchange footbridge at the north end of Farringdon station, and work on that starts next month.
Here are your before and after shots:
And here's the diagrams on the back of the leaflet:
Handily reoriented so that north is at the top. Thameslink trains use the tracks on the left, London Underground the tracks on the right.
The new footbridge will double as a new entrance to the station, which will come in handy as a lot is happening at the south end where the existing entrance is.
Update: Here's a much more detailed plan and an explanation from the planning application. A cross section is to the right - and until the trainshed roof is extended, a temporary fabric roof will be stretched across the top of the bridges.