I can think of many scenarios where a radar switches modes. But 'mode' is nothing more than a set of parameters which may include azimuth and bar scan settings, and radar PRF settings as well as antenna search speed.
Of course, there's HoJ modes, too.
In any case, the 'ATK' submode is basically what we call STT - and is largely a joke to use it as a meaningful reference in this situation.
Basically - Raptor's somewhere in front of you, you do an auto-acq search (all radars have such modes typically) lock onto it in no time, and proceed from there.
HOWEVER. AFAIK, most aircraft do not have the combined heater/gun 'Dogfight' mode like the F-16 does. Regardless, switching from gun to missiles is a matter of flicking a switch - it's quick, painless, and takes 0.25sec on average for you to do that
Despite this, the unfortunate truth is that a VC of 200kts and rising combined with the fact that seeker lock needs to be verified and the time it takes for the missile to come off the rail (which is significant, on the order of half second to a second) you're looking at losing some 1 to 1.5 sec of the time allotted, at which point you're under 500' distance which equals a MISS. Missiles have arming -and- guidance restrictions at such short ranges. THAT is why you have the gun.
Now, we could presume that the hornet could have fired its missile earlier - but in this exercise missiles were not a factor.
How can you tell? The hornet blew ALL its speed away to point its nose at the raptor for a snapshot with guns. In other words, a last hurrah.
Further, there's no flares in the images - and I'd say that lack of IRCM typically implies no missile simulation either. This was a gunzo fight.