But what few realised was that series creator Toby Whithouse faced a far greater challenge than simply replacing a single character - with Sinead Keenan (Nina) also declining an invitation to return for a fourth year and Russell Tovey (George) only signing up for a single episode, Whithouse was met with the prospect of completely relaunching a series that he'd guided since 2008.
Of course, from its earliest days, Being Human was no stranger to recasting - the original BBC Three pilot aired in February 2008 featured Guy Flanagan as Mitchell, W.E.'s Andrea Riseborough as Annie and, perhaps most bizarre of all, Hustle's Adrian Lester as the villainous Herrick.
But this was a different matter - could Whithouse hope to continue his show without characters to whom the audience had grown so attached? The answer, as anyone who's seen the exemplary fourth series of Being Human will know, is a resounding yes.
Given what we know now, it seems reasonable to assume that, with Tovey, Turner and Keenan all gone, Whithouse was eager to hold onto Lenora Crichlow's Annie for at least one more year.
The perky ghost's continued presence helped to smooth the transition as the show first upped Michael Socha's werewolf Tom to a series regular and then introduced Damien Molony's OCD vampire Hal. It was a canny move - a complete cast overhaul would've been too galling and may have alienated viewers.
Now that the viewer has grown to know and love both Hal and Tom, the sting of losing our last original cast member is softened considerably. It's been a gradual shift rather than a sudden one, lessening any feelings of resentment one might have felt towards the new cast. This writer certainly can't wait to see more of Kate Bracken's Alex in 2013.
With new blood came a fresh impetus and a realigning of the show's core values. Being Human still boasted some terrifically dark moments this year, but it was also brilliantly funny.
Molony's stuffy Hal and Socha's wide-eyed Tom are fantastic individually, but together they're unbeatable. Perhaps this writer's favourite moment from series four is Tom appealing to his friend's rigid sense of order - "Hal, I found these matches and some of them are the wrong way round!" he declares in mock panic. "We'll soon see about that!" retorts the vampire, gleefully snatching them away.
Yes, while much of the credit for Being Human's revival must go to Toby Whithouse and his team, it would be churlish to ignore the contributions of our new cast members. It helped that his character was already established and, in any case, couldn't be more different from Russell Tovey's George, but the newly-promoted Michael Socha nevertheless acquitted himself extremely well.
But it was perhaps Damien Molony - an unfamiliar face to television audiences, fresh out of drama school - who faced the more difficult task. Television critics and Being Human fans alike were sceptical that anyone could hope to fill the charisma void left by Aidan Turner, but Molony slotted in seamlessly.
From the moment this writer first met Molony on the Being Human set, it was clear that the 27-year-old Irish actor was a star-in-the-making - handsome, charming, yet incredibly humble and polite. Being Human should hang onto both him and Socha for as long as possible.
Another deserving of praise is Andrew Gower - certain to be overlooked by most, but a key component in the success of this series. As series two of Being Human proved, finding a villain to match Jason Watkins's Herrick is no easy task, but Gower's Cutler was a superbly creepy creation.
Come 2013, there'll be a new series of Being Human on our screens and we've got our fingers crossed that it can match the quality and vitality of this year's episodes. With a rock-solid new trio in place and Toby Whithouse still at the helm, we're confident that our hopes won't be in vain.
Do you agree that Being Human is now stronger than ever? Share your thoughts on series four, series five and beyond below!