The old trains are still entirely providing the service. The new trains haven't entered passenger service yet, indeed I don't think they've even tested them on the Subway itself yet, just the test track.
I can vouch for this about the old train sets - my Apple Watch keeps giving me warnings about excessive ambient noise on journeys between Buchanan Street and Govan. However, according to a staff member a single test train has been testing on sections of it (the “Sunday Closures’) since July. I do recall when the original Gen 1 trains were running and the tracks were realigned for G2, the G1’s could never be used again. I’ve never heard anything about this being a potential issue with the G3 sets. I’m sure there’ll be a press release when a date is set. It’s closed all day today for the last trial.
A spokesman is quoted: “Fault-free run testing is standard practice for the introduction of any new trains. If for any reason a train does develop a fault during the fault-free running, it is removed from the system while the fault is fixed and the clock resets on the 2000 miles so the process does take some time.
While these planned Sunday closures in July and August are to facilitate the fault-free run testing of the new trains, it is expected further Sunday closures will be necessary to continue the install of new communications and signalling for the new trains. These dates will be scheduled later in the year.
Passengers will be kept updated nearer the time via all our usual channels. Please follow @GlaSubwayTravel on twitter for the most up to date travel information.”