Arthur Lowe wasn't the first choice actor as Captain Mainwaring anyway. The part was written for Jon Pertwee. I suspect that Toby Jones might carry off the role rather well:
http://www.hulldailymail.co.uk/images/localworld/ugc-images/276270/Article/images/23424820/7321175-large.jpg
However, in such an ensemble piece as Dad's Army, it's important that
all of the roles are filled well. In the recent radio recreations of the missing episodes of Hancock's Half Hour, Kevin McNally was superb as Hancock but the other roles weren't filled as well, resulting in sitcoms which weren't quite up to the original standard. Let's hope that the new version of Dad's Army doesn't fall down for similar reasons (although the original movie version, rather than the TV shows, wasn't that great either).