On my 254 I only have to release two of the Allen cap head screws to use backgear, but it is still a weak point with no interlock to the speed selector lever, what would happen if this vital step was forgotten I do not know.
All your attachments tooling from the S7 will fit, vertical slides etc. are however mounted far from ideally, being way below their centre heights. The tailstock is a weak point, the barrel being the same as the S7 just in a larger casting. if using Dickson quick change tooling, you are restricted to rather small tools for such a large lathe.
On to the positive's, D1-3 camlock chuck mount is the best choice, the bolted on chuck option is fiddly to change, the power cross feed is good, the covered leadscrew is good, the speed range is good (bare in mind the silly back gear) digital readout scales fit quite easily. The topslide is more robust and far easier to use. All your Dickson tooling (if you have that type of toolholder now) fits. The extra centre height means making those traction engine flywheels are now possible or turning castings which are that little bit to large or heavy for the S7. The chucks are far more robust. The number 4 morse in the headstock is useful as is the larger through bore.