It'll take some time though. In case of routes like MMR-Pune or NDLS-GKP, the e-locos were being provided from NDLS side, and trains just had to proceed further from MMR or GKP with the same e-loco. So they could immediately start switching to e-locos.
But in this case, most of the trains in this route originate at SBC/YPR which have only trip sheds and cannot easily provide more locomotives on demand. There are about 30 odd trains on this route every week, and it'll take time for SCR/SWR to make arrangements to have more locomotives ready at GTL or SBC/YPR. So rest assured, it would take a couple of months on completion of the substation to...
more... have trains running with e-locos here.
The bigger problem is the incomplete sub-station. Unlike diesel routes which have practically no capacity constraints, Electrified routes are limited by the sub-station capacity. As the Someshwara substation is still to be completed, not many trains can run with electric traction as of now.
GY has only 2 platforms, that's why SCR doesn't do any loco changes there. Trains like Venkatadri and KCG-MS run with diesel traction between RU-GY . Dhone can surely have loco changes, so trains with scheduled halts will get a loco change there, but trains with only technical halts there like Raj/KSK will continue with diesel locos to save time.
SCR has traditionally preferred running trains with diesel-under-wire until the routes are completely electrified till convenient locations, particularly the twin-diesel hauled trains. So, the trains by-passing GTL and not having a scheduled halt at Dhone might not get e-locos until completion of the route till SC.