They had the same problem in the Aussie soap, "Home And Away", with Alf Stewart's daughter, Roo.
She appeared in the show for the first year or so, but then left & was never seen again, as the actress who played her, Justine Clarke, never wanted to return. She missed key family events, such as when her stepmother, Ailsa died in 2001 or when Alf had some health problems over the years.
They tried asking her back several times over the years, but the final straw was in 2008, when she didn't return for the 20th anniversary of the show & many other former cast members did.
So in 2010, they finally recasted the character with a new actress & most fans of the show that I know have got used to the new 'Roo'.
https://en.wikipedia.org/wiki/Roo_Stewart
So, with the new Michelle in "Eastenders", lets give Jenna Russell a chance & see how she gets on over the next few episodes etc. She is a also singer & sang the theme tune to the comedy series, "Red Dwarf". I thought I recognised the name from somewhere!...
I don't blaming them for recasting, as Sue Tully won't come back then it's fair enough. I hope they do the same with Zoe Slater, as Michelle Ryan has made it clear she dosen't want to return either, so they should recast her too!...
Some recasts work out in the end, as like many, I was a bit sceptical about James Bye as Martin in the early days, but now I think he's good in the role & I've totally forgotten about the other guy who played him all those years ago. Same goes for Ben too & I preferred Kim Medcalf as Sam, rather than Daniella Westbrook.