I don't think there have been any new special mission Y-8s built in years... All special mission/Gaoxin aircraft built in the last few years are Y-9 airframes (converted straight from the line of course).
But back when special mission based on the Y-8 airframe were being built back up to the late 2000s, I think they were converted straight off the line, and not rebuilds of old transports.
In fact, I don't know if any Gaoxin platforms or special mission Y-8/Y-9 aircraft were rebuilds of existing transports at all.... if anything, the Air Force's demand for tactical transports means they probably wouldn't be very keen on giving them up for conversion, and also putting in orders for new airframes means Shaanxi can get the much needed work and experience for building these aircraft as well.