Legacy AT&T customers were not happy about changes the company announced to its remaining data plans Friday. The nation’s second-largest wireless carrier said that it will slow the mobile Internet speeds of customers who pass into the top 5 percent of data users.

A quick search through social networks turned up several threats from AT&T customers saying they’ll switch to competitors because of the changes.

So what’s changing, exactly? Here’s a quick overview.

Who’s affected?: The plan changes only affect users who opted to keep their unlimited plans when AT&T stopped offering them to new users in June 2010.

When: The new plan limits will go into effect Oct. 1.

How much data?: The company said that it will throttle back data use for the top 5 percent of data consumers, who use “twelve times” what its average smartphone data customers use.

A recent Consumer Reports survey found that the average smartphone user on AT&T’s network uses 360 megabytes per month — meaning that only power users will feel the pinch. Using AT&T’s formula, the company’s likely scaling back its network for users who exceed 4 gigabytes per month.

So ... how much is 4GB?: This is a question that came up when Verizon moved to tiered plans early last month. AT&T’s data calculator indicates that users would have to watch over an hour of streaming video a day to hit 4GB per month.

How will I know how much data I use?: AT&T said that it will be notifying customers with “multiple notices” and a grace period if they’re nearing the new limit. And, the company said, it will restore data speeds at the start of every cycle.

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