I watched a few of the first episodes of the new 'Robin Hood' a couple of seasons ago, and just didn't like it. It wasn't so much that I already had a 'Robin Hood' - for good or ill - established in the main Toobworld; it was because the writing was anachronistic and at times it was reduced to the pie-throwing level of a Stooges short.
Season three will begin airing this spring, and it's already been announced that Jonas Armstrong, who plays Robin Hood, will be leaving the role at some point during the run. They've already lost Marian to Guy Gisbourne's sword, and now several other actors (including Harry Lloyd, whom I liked a lot in the 'Doctor Who' two-parter of "Family Of Blood" & "Human Nature) are planning to exit as well. Although not confirmed as of yet, Richard Armitage (who plays Guy) may have to leave as well as he's joined the cast of 'Spooks' and that's been re-commissioned for another season (series if you prefer).
And yet, even with so many of the original actors abandoning ship, there are still talks for a fourth season to be made!
The last major version of 'Robin Hood' on television had a similar casting challenge when Michael Praed left the series and was replaced by Jason Connery. But as I wrote this morning to The Stage's TV Today column:
At least with that earlier version of 'Robin Hood' with Praed and Connery, they established that they were two different men - along the lines that one was Robin of Sherwood and the other Robin Locksley, something like that.
And like you said, they had the benefit of keeping everybody else from the cast.
With such an overhaul of the actors about to happen with this version, I think they should find a voice-over artist who sounds like the guy who used to do 'Batman' back in the 1960s. And they should kick off Season 4 with one last look at the original cast and then the announcer says, "Meanwhile... in another dimension....."
William Dozier, the producer of 'Batman', also served as its narrator. Unfortunately, he passed away in 1991, but a sound-alike would make for a great kick in the tights!