Is this a $100K condo, or is it worth $1M?  Is the client in a 15% bracket, or 35%?  Has there even been any tax benefit from the excess depreciation, or is he at zero tax anyway, or with a carryover loss because of higher income?

If you question is "What is required to follow the letter of the law?" see @itonewbie .  If your question is "what will happen if I just start doing it the right way now?" the answer could be "not much."

“If the law supposes that," said Mr. Bumble, squeezing his hat emphatically in both hands, "the law is a ass — a idiot. If that's the eye of the law . . . the worst I wish the law is, that his eye may be opened by experience — by experience.”  --From Oliver Twist, by Dickens