Freeview users will need to retune their set-top boxes and TVs on 30 September as the service is upgraded.
The move aims to give more viewers access to channel Five.
However, those who have channel Five at the moment will be unable to watch it until they have retuned. Some older set top boxes will cease to work at all.
ITV3, ITV4 and some BBC radio stations will also be affected. About 460,000 households are expected to permanently lose access to ITV3 and ITV4.
The technical changes behind the upgrade will eventually enable high- definition (HD) television to be broadcast on Freeview. HD is already available on the free-to-air satellite service Freesat.
However, the change threatens real confusion for millions, particularly the elderly, who may not understand how to retune their sets. Electrical retailers say many customers are unaware that their Freeview equipment will stop working unless they retune it.
The changes to the transmissions, which will take place on Wednesday morning, will also mean that 22,000 older digital TV set-top boxes will no longer work.
From September 30 there will be some 50 TV channels, including mainstream ones, Film 4, those from Sky and Virgin, and various shopping services.
A new channel called Quest – a mix of factual, lifestyle and entertainment programmes from the archives of the Discovery Channel – will be introduced.
The switch is being organised by the Freeview organisation in partnership with the BBC and the commercial channels.
It does not apply to viewers who get their TV and radio channels from set-top boxes linked to Sky TV or Virgin Media.
Nor will it apply to set-top boxes and TVs tuned in to the Freeview satellite system.
One independent Scottish electrical retailer said: ‘We have been amazed at how few people know what is going to happen.
‘This national retune may cause problems for some people.’
Freeview said the changes are necessary to ensure wider access to Channel Five which, despite being a national channel, cannot be viewed in many parts of the country.