Moving into an empty territory with an HQ is an expansion with no troop loss.
I'd assume if it was a major-city abandoned HQ (or worse yet, a major city with a fortification and an abandoned HQ) that would make it an expansion with troop loss?
Yes. It was very early when I typed that.
A HQ and NO city is an expansion with no troop loss.
Otherwise you would expand and lose troops as per the standard rules for an empty city.
I had to dig it up, because I really was not sure which interpretation was right...