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If you already have acceptable received signal strength (as indicated by rersonable signal quality on your receiver or IDTV) then it won't make any difference.
If, however, you are subject to occasional dropouts, pixellation etc. chances are that these will be eliminated.
When they switch off the analogue signal, they greatly increase the signal strength of the digital signal. That enables people who were previously living in marginal signal areas to get a picture ok.
I know that on the Stockland Hill transmitter at least (covers Exeter, East Devon, West Dorset) they also switched on the digital signal at all it's smaller, infill transmitters at the same time. Previously, some people were unable to get digital until this point.