Conforming the original mono to the extended cut, also in mono, should be relatively simple. Filling in those sections on the multi-track may not be as easy.
On the Psycho 4K, supposedly what's billed as "English 2.0 Mono Original Audio" is a fold-down of the new multi-channel mix, which had revised sound effects added to it. If that's the case (I've not heard the new disc) then it's cause for a replacement.
And I don't have the new Blu-ray, but I...