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And changed from a comet to a planet in that span of time (around 3,000 to 4,000 years ago)? Did it transform while being ejected from Jupiter? Or halfway to its current orbit? Or all the way to its current orbit?

Where did it pick up ALL those orders of magnitude extra mass? HOW did it overcome the ejection velocity's tendency to shed mass? What made it stop where it is, especially after having acquired all that extra mass?

And, most importantly, if this can happen, why hasn't it happened again? Why aren't we observing it happen regularly with other comets within the outer solar system?

AHHH, you're exposing the lie
The concerted effort against Velikovsky was to declare that "comets" were tiny, fluffy snowballs.
and therefore not planet sized bodies made of rock and stuff
As Laird has said, the ancients, without the advantage of this new definition,
saw in the sky, a big hunk of stuff with a tail, and called it a comet

The insistence and effort on maintaining the "new definition"
(made before any probes were sent to the comets)
in spite of all new data showing it is wrong
is simply amazing

Whether Velikovsky is right or wrong
is immaterial to the fact that the probes
to the comets have all shown that the
fluffly snowball/icy wanderer/dirty snowball/snowy dirtball/ model is wrong
however you try to move the goalposts