Changing an offpeak return ticket so I can return in peak time.

I'm guessing it's too late to provide an answer that's of use for yesterday's journey. As I was out all day I didn't have the opportunity to look at this query. Sorry.

But for next time, and as a general reply, this post on another forum says it all:

http://www.railforums.co.uk/showpost.php?p=1185795&postcount=6

Quote

4.2.4 Excess procedures: change of ticket type

Change of ticket type excesses allow a journey to be made at a time when time and/or break of journey restrictions prohibit this. There is no penalty for buying on board even if there was opportunity to buy before boarding.

If bought from ticket office or on board with no opportunity to pay before boarding:

For Singles & Returns, the whole difference is charged
For one portion (Outward or Return) of a Return ticket, the FULL difference is charged. Alternatively, the passenger can buy the APPROPRIATE Single instead.
The difference is between the price paid and the APPROPRIATE ticket type of the route requested.
Railcard discounts ARE available

If bought on board where there was opportunity to pay before boarding:

As above: there is no penalty on board with this type of excess.

In more detail. The specifics of your fare:

Thatcham to London Zones 1-6 Off Peak Day Travelcard ^26.60
Thatcham to London Zones 1-6 Anytime Day Travelcard ^54.70
Excess Fare ^54.70-^26.60 = ^28.10

Remember though that with Off Peak Day tickets to Paddington (incl. Travelcards) you are only barred from using mostly longer distance services off Paddington in the evening peak for your return journey. There is a specific list of barred trains. They are (as of February 2015):

From From
PAD^» (Paddington (London) - next trains) RDG^‡(resolve) Final Destination
1600 1627 Bristol TM^‡(BRI^» )
1606 1633 Penzance (PNZ)
1615 1641 Swansea (SWA^» )
1630 1657 Taunton (TAU^» )
1636 1704 Exeter St D(EXD^» )
1645 1711 Swansea (SWA)
1649 1720 Oxford (OXF^» )
1700 1726 Bristol TM (BRI)
1703 1732 Penzance (PNZ)
1715 1741 Swansea (SWA)
1722 1750 Hereford (HFD^» )
1730 1756 Taunton (TAU)
1733 1804 Paignton (PGN)
1742 n/a Cheltenham (CNM^» )
1745 1811 Swansea (SWA)
1749 1822 Worcester (WOS^» )
1800 1827 Bristol TM (BRI)
1803 1830 Penzance (PNZ)
1815 1841 Swansea (SWA)
1822 1850 Hereford (HFD)
1830 1856 Weston SM (WSM)
1835 1902 Exeter (EXD) FX
1835 1902 Plymouth(PLY^» ) FO
1845 1911 Swansea (SWA)
1847 1918 Cheltenham (CNM)
1850 1922 Oxford (OXF)
1900 1927 Bristol TM (BRI)
1903 1933 Plymouth(PLY) Fridays excepted
1903 1933 Penzance(PNZ) Fridays only
1915 1941 Swansea (SWA)

Most of those won't apply to a direct journey to Thatcham. Essentially you can travel to Thatcham with an Off Peak ticket on any direct service in the afternoon/evening. That means you're fine with an Off Peak Day Return/Travelcard on the 1618, 1706 and 1805.

In summary, what this all means is that you only really need an Anytime Day Return/Travelcard if you are travelling to London in the morning peak. Or you desperately want to travel fast to Reading in the evening and then change there for a service that starts at Reading for Newbury/Bedwyn. You can of course take any 'slow' service off Paddington (i.e. any one not on the above list) to connect at Reading with a service starting there for Thatcham.