I recall taking a brand new Poseidon Odin on a Great Lakes wreck dive for its inaugural dive. Switched out either a SP D400 or G250 for the Poseidon. Although the Poseidon functioned okay, it breathed like a dog compared to the SP. After I voiced my displeasure during my surface interval, one of the divers with me (I can't recall now whether it was the owner of my LDS from which I purchased the Poseidon, or the captain of the six-pack we were diving from, who dove Poseidon exclusively) took a few minutes to adjust my new Poseidon's second stage. The Poseidon breathed like a dream, down deep, during my repetitive dive.
Upshot: I agree with the people above who suggest that it's prudent for you to first "function-check" your "new" reg (that's been sitting awhile, unused), and then take it for a "real" dive somewhat before you leave for your dive vacation. Leave yourself enough time so that your reg can be adjusted and function-checked and dived again after the service, before you leave for vacation.
rx7diver