+1 for rocklebock's consideration. What are your career aspirations? What are the promotion possibilities in your current department? What are the promotion possibilities in the new department? Weigh all of that and see if the jump still makes sense. I would discount the boss trying to talk you out of it; it sounds like sour grapes.
Based on what you've posted, I say go for the interview. Until you get a job offer, there is really no decision to make.
Thanks for the thoughts everyone. Lots to consider.
To the point raised by Jim and rocklebock: it may be harder to move up within this particular unit, but opportunities at other units are always posted and are always available to all staff. That doesn't mean I'd have the same chances as an internal candidate, but the opportunity is there.
However, I don't want to grow my career in this particular department. Those crazy scheduling demands increase as you advance here. The salary increases, but my boss makes around $90K and works far longer hours including frequent e-mails at night and on the weekends with no overtime OR even comp time. To me, that is not worth it, especially when you can make the same or more at another state agency with very little overtime. This is one of the main reasons my agency experiences a great deal of turnover.
The new job seems like a good place to spend some time growing my salary (we get annual advances plus annual 2% COLAs - and yes, I'm in a trad pension plan) and learning some new skills. Obviously it comes with the very important side benefit of more work-life balance.
I had the interview today and I think it went very well. The work seems interesting enough but more importantly the team (who I knew casually already) seems great. Very nice people. Hoping I get invited to a second interview with the unit director.