According to Radio Times,

this new version of the serial will feature a ‘lost’ piece of Doctor Who history – while the Second Doctor’s regeneration into the Third Doctor (Jon Pertwee) originally took place off-screen, the special edition of The War Games will depict the changeover on-screen.

and

We’re also promised the new release – which will air on BBC Four and be available on BBC iPlayer – will feature “recovered footage not seen since the original broadcast”.