This is a periodic major overhaul, including repair to bodywork so the unit would need repainting after the work. In addition, the 3 car 158s were some was of the very first Arriva repaints with 752 being done in September 2016, so after 8 years a repaint is due anyway.
If you're taking everything out for overhaul you might as well freshen the interior up - some of the 3 cars never got fully painted internally too.
I don't know the specifics of the work, I suspect the original seating will be retained unless it's cheaper to replace.
@diligentdave will probably know though.