Of course much has already happened, with the old concourse and roof
next to the terminating platforms magically being transformed into the
new (and the disappearance of the distinctive 1970's brown canopy edging
[1]), but this weekend sees some significant changes with the first two
new platforms being brought into use...
http://www.thameslinkprogramme.co.uk/node/121

Many changes to services as a result, including no Southern of FCC
trains in or out of London Bridge between around 10am and 11:30am this
morning, nor any on Saturday or Sunday (FCC Thameslink goes via Elephant
& Castle instead, whilst there are major changes to Southern services
[2]). Southeastern services will still run but there are some changes...
http://www.thameslinkprogramme.co.uk/node/120


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[1] http://www.flickr.com/photos/24255527@N00/2465215262/

[2]
http://www.southernrailway.com/your-journey/plan-your-journey/improvement-works/