First step - a plan. Following in others' footsteps is a great help especially when they share their resources. So I have Diagrams of the LSWR saloon (1869- remodelled 1889) and a 4 compartment coach both in the 24' 6" length of #17.

Ratio 612 (a Diag. U4; 26' 10" composite) is a suitable starting point, the main issues being to shed the extra width of the First Compartments and create the central saloon. Unlike #15 where saloon had its own entrance and no thoroughfare, #17 had access from the compartments at each end as per #1520 on the Bluebell Railway

LSWR 1520

From Wikipedia Commons - at the above link

Following on from the blueprint, the first step is to carefully cleanup the surplus items on the moldings, in my case the door knobs and grab rails as well as some flash. This is shown below with just one more door's fixings to be trimmed.
 

The bottom pair shows side 1 after all the required cuts and several joins have been made. The "X" marks where the other Saloon window will be created by cutting away the current panel area. Minor off-cut inserts are still required to complete the first saloon window frame.The as yet uncut other side is shown for comparison, 9mm longer than the end product.


Compared to my other cut 'n shut on #15 this is relatively easy because all the side components are from the same source. #15 used sections from 2 different Ratio Kits which were a poor match and caused a lot of issues.

Another useful reminder to self, when assembling side #2, mirror it along side the first by inverting as shown here:

Particularly helpful when the sides are asymmetric as in this case otherwise it's hard to visualise the reverse assembly order for the second side. Here side #2 just needs the missing piece of the jigsaw puzzle to be cut to fit.

The remaining tasks are now done, the spaces for the two saloon windows each side were cut out completely and a new upright for between the two panes cut from the surplus stock to hand. These are best fixed in place with a thick paste made from surplus sprue and limonene, about the consistency of toothpaste. This has a good combination of features - gap filling, high tack but enough give to enable careful positioning.

I'm lucky to have one decent photo of #17 showing that after refitting for WC&PR service the ventilators ran the full length of this coach, unlike the others. Some fairly soft 0.5mm PS sheet was perfect for this job, easy to score at close separation. The insides of the coach walls were reinforced with a 0.5 * 3mm strip of more rigid PS.

All major components have been reshaped to the required sizes and just a few notes here for future reference.

Sides: An omission in my #15 assembly was to add additional material to the ~1mm flooring attachment strip. Here a second 1mm * 1mm PS strip has been glued on top as this gives a very positive surface to glue the side to the floor. It can be seen below the 3mm strengthening strip I'd added earlier.

All door furniture holes were pre-drilled before priming

Sub-chassis However carefully measured out it's an immutable law of nature (in my universe anyway!) that it won't be perfect. I think my cutting + filing are to blame for leaving the floor 1mm short after gluing back together. This is only obvious when put together alongside the cut down sides. No matter, a simple 1 * 1 mm PS strip was glued on one end to make up the shortfall. Some kits can have variances of this magnitude # and it's always worth checking before gluing!

Ends: faux handrails were trimmed away. This is tricky as they are barely 0.3mm above the surface, best erring on side of underdone and then errors spotted when priming than gouging too deeply (note to self) which is hard to repair

Roof: even with 9mm cut out, no need to rework the rain Strips as the match was fine. Priming shows up any minor gap filling needed at the join.

Note re image reload Apr 22: These lost images were reloaded, some now display inverted but that is a step to far for me to fix up.