Actually, it wasn't as bad as it looks from the outside, more a hazard in ground ops. than airborne. The middle pane was a downward hinging DV window. Only the first few dozen 700 aircraft were built with this, the remainder being built with the single pane sliding DV window - as you say conversion to 700C would have taken care of this mod.
The only other outward signs of the later models was the disappearance of the "Donkey's Dick" airscoop on the lower rear fuselage, replaced with a flush model.
The props changed too from rounded tip to square tip when the more powerful Dart Mk.510 replaced the Mk.506 and lower as illustrated in the two photos above.
The instant giveaway on the ground when taxying in was which pair of engines were running, the early 700s had hydraulic pumps on the outboards whilst the later ones and the 800s had them on the inboards, a good point to watch when flying both types to avoid embarrassment.