They might now appear as channels starting in the 800 range, as most decoders re-allocate duplicates up there.
Note that the changes from Winter Hill mainly affect the BBC transponder on Channel 62, which has now moved down to 50 something, if I remember correctly.
The usual industry web pages recommend a complete re-insatllation to be performed on your digi box or tv, rather than the 'add channels' option.
As the Dr suggests, complete or partial removal of the tv aerial plug will reduce the signal, so that the stronger signals are seen and get allocated first.
As long as most the other channels are still there, you don't need a new box or aerial, so don't get fooled into parting with any money on something you don't need. Obviously, if a service engineer is called, then he will want paying - but really, it should only be a simple job that you can do from the comfort of your chair, using the remote control.
Take a look at http://www.bbc.co.uk/reception/digitaltv/
for more help and advice.