Can the negative effects of smoking be completely reversed over time if you quit?

Yes and No: It appears the "normal" decline in lung function which occurs for all of us can be obtained within 6 months after stopping. Effects on taste, gum disease, and even cancer risk can expect to be "normalized" after stopping. Nicotine receptors never down regulate and highly dependent tobacco patients benefit from long term nicotine replacement. Structural tissue damage is likely irreversible.