The 800 is probably an easier one to solve than the 700, and it could be done without any loss of capacity - all it needs to solve the main problem is to remove the seat covers and foams, bin them, and replace with slightly thicker, softer, differently shaped (fire retardent) foams and more colourful covers, and stick a dark green plastic stripe over the nasty 1980s-bus-style lime green one. In the great scheme of things it would not be expensive. From an engineering perspective, while it isn't anything super-special it is a reasonably solid train which will last well, it just has an issue with the seating.
The 700 is probably harder to solve, as to solve it properly you'd probably want to get rid of 1st, fit 1st style seats in more of the train and fit side-facing seats elsewhere to maintain the standing capacity. It wouldn't even be possible just to remove some luggage racks and respace the seats because the door pockets narrow the saloon too much to just move a row back[1]. Though as per the 800 there's nothing fundamentally wrong with it other than the seating.
[1] Or do they? The Class 319 PRM-TSI work moves a row back towards the luggage rack overlapping the door pocket to make a priority row, and it just protrudes out another 1.5" ish into the aisle than the other rows.