Well we can do some intelligent guessing. Its not so much of a change to take it back to the design stage (something we saw with Ra'ad integration and plaque
@JamD brought to our attention). Similarly it is not too minor something like a software update, which could happen even after handover.
Now, I would base my guess on three observations, no.1 it is around the time J-10s have come. No.2 having noticed 3P03 to have gone back into final assembly without receiving PAF colours while later no.s are in PAF colours (see PTV World program) and No.3 Panzer mentions they are conducting taxiing tests on it, we know DER with SD-10s have already gone through that (there was a picture months before
@Deino knows), so I would hazard a guess it is just testing if they can put Pl-15s on DER. Maybe their original plan was to have it Blk-3 carry only 2 but now they suddenly find a good enough inventory costs to make carrying 4 more feasible.