All depends on how you define remodel - the station is one thing, the signalling and track layout through the Oxford area another.
Work to clear trees from the formation of the former down goods loop (running north from Aristotle Lane bridge to Wolvercot) is under way now, with work going on the rest of the year to get it back into shape, sort out the drains and lay track. This is in connection with work to allow 775m container trains to run between the Midlands and Southampton, with passenger trains using the loop lines and freight going down the middle pair of tracks. See
http://www.oxfordmail.co.uk/news/10211103.Sadness_as_Wolvercote_trees_go_to_improve_rail_links/
An intermediate phase of resignalling is also going ahead shortly to prepare for electrification, with control switching to Didcot, then there will be more signalling work and serious changes to the track layout as part of the station redevelopment, plans for which are still being drawn up, with the rail industry and the city and county councils all involved.
The station and Oxford area capacity enhancement was mentioned in the HLOS last summer, so some level of funding is already allocated, but until the scope of what will be done to the complete track layout, buildings, station entrances and other facilities, how much money will be needed in total remains to be seen. The councils will clearly be bidding for grants from the government at some stage.
Back on the main premise of this thread, I gather contractors of some sort have taken over much of the car park at Islip station and the legal challenge to the DfT decision on the granting the order to Chiltern is likely to get to court next month
http://www.oxfordmail.co.uk/news/10226360.Sole_campaigner_objects_to_London_rail_link_plans/