It's likely your client will still maintain his rankings for "city + service" searches since the city is not changing, but, remember, your client does have competitors out there working on local search too. Regardless of your street address change, rankings may fluctuate based on how hard they're working. A drop in rankings would occur if you sit back on your laurels and all of your competitors are working tenfold (naturally, organically) on local search, but I don't think changing a street address would have a major effect.
I hope someone on this thread will comment about their tested results with having to do the same thing, since I don't have direct experience here.
To add, you absolutely have a big task at hand to make sure all of the downstream listings that feed into the local search algorithm (and Local+) are all corrected to reflect the new street address.
Maybe check out this link from Blumenthal's about moving a business to a new location: http://blumenthals.com/blog/?s=changing+address
Also, the readers at Blumenthal's are also very committed to giving advice and answering q's. Might be another resource for you on this question.