Heh, I deduced a way it might be possible, assuming sufficient coordination.
I don't know specifics of radar, just the general principles. The major possibility here is that your suggested transfer of which transponder is saying "I'm MH123 at the same altitude" happened at a range where no radar would be giving a good enough return from the putative small jet assuming the role. Which upon review wasn't the scenario you were positing.
And if it was giving enough of a return to be seen, especially in retrospect as experts reviewed it, well, we'd have heard about it by now.
But if AB123 was big, we'd really have heard about it and it's near collision with MH370, or someone would be wondering why AB123 moved really quickly a noticeable distance at the same time MH370 went off the air. Or perhaps AB123's altitude was different, they were far enough the radars' couldn't tell that, and it's transponder was customized to lie about the altitude.... But people would still wonder, and would be questioning AB123's flight crew if they saw anything, etc. etc.
(I'm tired enough the above isn't entirely coherent, but it'll give you some things to chew on.)
Again, I would like to stress that I know all of this is unlikely, but there's is no reason why both planes would need to coordinate this down to the second. Mh370 goes offline, takes an hour to get into position with ab123. Then they make the switch.
Once mh370 goes missing, it stays missing. The trick is in making it look like ab123 is behaving normally. And while everyone is distracted by the search for mh370, I wonder how closely they would be looking at irregularities in other flights that aren't missing and aren't reporting any problems.
I don't know specifics of radar, just the general principles. The major possibility here is that your suggested transfer of which transponder is saying "I'm MH123 at the same altitude" happened at a range where no radar would be giving a good enough return from the putative small jet assuming the role. Which upon review wasn't the scenario you were positing.
And if it was giving enough of a return to be seen, especially in retrospect as experts reviewed it, well, we'd have heard about it by now.
But if AB123 was big, we'd really have heard about it and it's near collision with MH370, or someone would be wondering why AB123 moved really quickly a noticeable distance at the same time MH370 went off the air. Or perhaps AB123's altitude was different, they were far enough the radars' couldn't tell that, and it's transponder was customized to lie about the altitude.... But people would still wonder, and would be questioning AB123's flight crew if they saw anything, etc. etc.
(I'm tired enough the above isn't entirely coherent, but it'll give you some things to chew on.)