On First Great Western, the 07:06 London Paddington to Paignton is diverted to Penzance, and the 07:30 London to Penzance via Bristol runs to Paignton in it's place. There are other tweeks in the South West to improve connections etc.
Generally its to improve the service to/from Devon and Cornwall especially filling in the current gap in London service from Penzance between 1000 and 1400, and also on the Berks and Hants route between Pewsey and Castle Cary to/from London.
In Devon and Cornwall the 1400 from Penzance now starts Plymouth connected into with a unit, with the 1500 from Plymouth starting at Penzance at 1303. This then means the 4 hour gap is reduced to two 3 hour gaps.
Meanwhile on the Berks and Hants the current 1218 Paddington to Taunton will cease to exist replaced with a Turbo to complete the hourly service which currently has 1 HST in its path each way during the day.
As a result of this a new service is created, an 1133 Paddington to Exeter this calling Newbury, Pewsey and Westbury on the route allowing the removal of the Pewsey stop on the 1106 Paddington to Plymouth. This service then forms the current afternoon stopping service from Taunton to Paddington starting at Exeter.
These changes also allow the current 1106 Paignton to Paddington to run later (and therefore being formed from 0730 from Paddington), and as previously mentioned no longer calling at Bedwyn, Hungerford, Thatcham and Theale. (This service would of passed the current 1218 Paddington to Taunton at Bedwyn, to form what will now be the next service for the extra Turbo service)
Note though that these alterations are only on a weekday.