Looks like the Reading area overhead wires will go live for testing on the weekend of September 16 and 17.
Very limited GWR service through Reading on the Saturday until 14.00 - then diversions for South Wales, South West and Bristol services via Oxford, Bicester Village and the Chiltern line and buses in Thames Valley apply for the rest of the weekend.
Trying to decipher the GWR web page and what is on realtimetrains, looks like HST calls at Oxford station on the diversions may only be for crew change purposes, not to pick up and set down passengers, with Oxford-London traffic directed on to Chiltern.
Details of alterations here
https://www.gwr.com/travel-updates/planned-engineering/reading2017