Leonard Rossiter "replaced" by Bruce Forsyth Tripper's Day->Slinger's Day
Sitcoms with major cast changes Page 2
Rimmer was replaced with Kochanski when Chris Barrie left.
The old guy was replaced by Gunther in Central Perk.
Leslie Scofield replaced Tim Preece as Tom Patterson in The Fall & Rise of Reginald Perrin.
Daphne Reid replaced Janet Hubert as Vivian Banks in the Fresh Prince of Bel Air.
Slightly different but - Christopher Ryan replaced Keith Richardson for the part of Mike in The Young Ones.
Def.
Moose was replaced by Enzo as Eddie in Frasier.
Quote: Afinkawan @ June 16 2008, 1:48 PM BSTMoose was replaced by Enzo as Eddie in Frasier.
They replaced Eddie?!?
Peter Bowles replaced Geoffrey Palmer in Executive Stress
Richard Wilson "replaced" Geoffrey Palmer in Hot Metal
Donald Churchill replaced Ronald Fraser in Spooner's Patch
Simon Cadell "replaced" Roger Rees in Singles
David Griffin "replaced" Simon Cadell in Hi-De-Hi
Ernest Clark replaced John Barron who returned to replace Ernest Clark in All Gas & Gaiters
Grham Bickley replaced Peter Howitt in Bread
Melanie Hill replaced Gilly Coman in Bread
Erika Hoffman replaced Kim Thomson in Brush Strokes
Gary Webster replaced Dennis Waterman in Minder
PS Christopher Ryan replaced Peter Richardson for the part of Mike in The Young Ones.
Tony Steedman replaced Peter Vaughan in Citizen Smith
Ian Saynor replaced Lewis Collins in The Cuckoo Waltz
Ingrid Lacey "replaced" Haydn Gwynne in Drop the Dead Donkey
Karl Howman replaced Robert Lindsay in Get Some In
Keith Drinkel replaced Stephen Rea in I Didn't Know You Cared
Liz Goulding replaced Anita Carey in I Didn't Know You Cared
Carmel McSharry "replaced" Dandy Nichols in In Sickness and In Health
Sabina Franklyn replaced Jenny Quayle in Keep it in the Family
Anita Carey "replaced" Frances de la Tour in A Kind of Living
Liz Estensen "replaced" Polly James in The Liver Birds
Lesley Dunlop replaced Eve Matheson in May to December
Neil Morrisey "replaced" Harry Enfield in Men Behaving Badly
Peter McEnery replaced Jack Galloway in The Mistress
Barbara Murray replaced Nyree Dawn Porter in Never a Cross Word
Andrew Charleson replaced Martin Clunes in No Place Like Home
Quote: Alan C @ June 16 2008, 2:16 PM BSTTony Steedman replaced Peter Vaughan in Citizen Smith
Ian Saynor replaced Lewis Collins in The Cuckoo Waltz
Ingrid Lacey replaced Haydn Gwynne in Drop the Dead Donkey
Karl Howman replaced Robert Lindsay in Get Some In
Keith Drinkel replaced Stephen Rea in I Didn't Know You Cared
Liz Goulding replaced Anita Carey in I Didn't Know You Cared
Carmel McSharry "replaced" Dandy Nichols in In Sickness and In Health
Sabina Franklyn replaced Jenny Quayle in Keep it in the Family
Anita Carey replaced Frances de la Tour in A Kind of Living
Liz Estensen "replaced" Polly James in The Liver Birds
Lesley Dunlop replaced Eve Matheson in May to December
Neil Morrisey "replaced" Harry Enfield in Men Behaving Badly
Peter McEnery replaced Jack Galloway in The Mistress
Barbara Myrray replaced Nyree Dawn Porter in Never a Cross Word
Andrew Charleson replaced Martin Clunes in No Place Like Home
I was just about to post those ones, but you beat me to it
Ronald Hines replaced Paul Daneman in Not in Front of the Children
Lynda Baron replaced Rosemary Martin in Oh No It's Selwyn Froggitt
Donald Sinden "replaced" Leslie Phillips in Our Man from (at) St Marks
Terence Brady replaced Dinsdale Landen in Pig in the Middle
Jonathan Cecil & Gay Soper "replaced" James Beck & Jo Rowbottom in Romany Jones
Daniel Ryan replaced David Michaels as Harry in As Time Goes By.
Alfie Bass replaced Arthur Brough in Are You Being Served?
Larry Martyn and Graham Stark replaced James Beck as Private Walker in the radio version of Dad's Army.
John Quayle replaced Tim Barrett who had replace Terence Alexander in Terry and June
Wallas Eaton replaced Max Adrian in Up Pompeii
Henry McGee replaced Percy Herbert in The Worker
I've ran out of letters so I must be done 
Quote: Greg @ June 16 2008, 2:21 PM BSTAlfie Bass replaced Arthur Brough in Are You Being Served?
And was in turn "replaced" by James Hayter, or was it the other way round?
John Alderton in the Upchat Line was "replaced" by Robin Nedwell in the Upchat Connection.
Was not Ronald Lacey’s Harris a replacement for Ken Jones’ Ives in Porridge? In the film version of Porridge Geoffrey Bayldon replaced Michael Venables as the Govenor.