How does one determine why a plant's scientific name has been
changed in recent years? Is there some journal (online or not)

This is why serious gardeners become babbling idiots. There are two main
reasons why a plant name gets changed. One is the law of priority. A botanist
does his homework & finds that a supposedly recently discovered plant was
actually named years ago. In that case the older name must be used. The other
main reason is that a plant may have been assigned to a certain genus, but
subsequent research leads to the genus being broken up, or a species being
transferred from one genus to another. This is the main resource I use:
http://www.ipni.org/ipni/query_ipni.html
buth there are others. See the mini-FAQ.

the official record-keeper of these changes?

For flowering plants it is Index Kewensis.

Iris,
Central NY, Zone 5a, Sunset Zone 40
"If we see light at the end of the tunnel, It's the light of the oncoming
train."
Robert Lowell (1917-1977)