Fact: yes
Janet is replacing all of their 737-200s with -600s, supposedly from a Chinese airline. I believe this is their only jet so far.
I have two buds who fly for them (and they want me to join as well..becoming very tempting). All the -200s will be parked by next summer; the -600s are replacing them one-for-one.
The -600s are unique in that they are being modified to have the -200's "folding-ladder" airstair, as Janet flies to some places that may not have jetways or airstairs.......
They need the -600 as they are often working against a very significant density-altitude problem for takeoff (not Vegas...the other places).