I would think it could be weird for Neil and Mike to sing and play Lindsey's parts, and even just to be in Fleetwood Mac. It's not that much different to being in a Fleetwood Mac tribute band! Neil's a fine songwriter, and now he'll be performing someone else's compositions. I wonder if the gig is just for touring, or if Neil and Mike are permanent members and will be recording new material with the group. Fleetwood Mac's fourth incarnation?! Or is it the fifth? Still my favorite lineup is that of the Kiln House album era. For guitar players it's the original, Peter Green lead version. I have been wondering what Mike Campbell would do post-Petty, and now have the answer.
Of course I was a HUGE fan of the Buckingham/Nicks era Fleetwood Mac, but recently went back to revisit the early stuff.  I'm not a fan of the first three albums, but from Kiln House (1970) to Future Games (1971) to Bare Trees (1972) to Penguin (1973) to Mystery To Me (1973) to finally Heroes Are Hard To Find (1974) these albums are pretty incredible.

IMHO, these era albums are overlooked/underappreciated because they weren't big commercial successes like the later works. 
I'm a fan of the original four (and later five) piece band with Peter Green, Jeremy Spencer and Danny Kirwan on guitars.  That band had an amazing sound.  I sometimes hear traces of it in the music of Los Lobos and ZZ Top.  Lots of talented musicians came after Peter Green left but I don't have much interest in what they did.  It is interesting that the name survives--the music is so very different.
I'm firmly planted in the Peter Green incarnation. But it's amazing that they've been through so many transformations, most of them very, very good, and that they continue today!
I’m with ya’ Mofimadness, all those lps you like starting with Future Games, had Bob Welch as I am sure you know. He never got the credit he deserved for leading FM with Christine McVie in the mid-Mac era. Bare Trees and Mystery to Me - not a bad song on Mystery, even the Yardbirds For You Love is great. Well recorded, layers of guitars, good vocals. He started the transition into what they became. Welch story is so sad- committed suicide so his wife would not have to care for him due to severe back issues post surgery. For me, Then Play On is a classic - serious organics while listening to that one during college. Still play the first and second Mac periods often!