To begin with, AT&T will be bumping up the cost of their basic unlimited data plan, dubbed "Unlimited Choice Enhanced," from $60 to $65 per month. The company will also be dropping the price of their higher-tiered, "Unlimited Plus Enhanced" plan from $90 to a mere $80. To be clear, these price changes are only going into effect for new unlimited data plan customers; existing customers will not see any changes to their monthly bills.
At any rate, these price adjustments come with a few service tweaks. Specifically, AT&T's Unlimited Choice Enhanced plan has had its 3Mbps speed cap for "overall Internet usage" eliminated.
Theoretically, this means customers will get faster data speeds in general with the exception of video playback which will remain locked to 1.5Mbps for 480p content. Unfortunately, AT&T will also be throttling Unlimited Choice Enhanced subscribers' connection speed if the company ever feels their network is too congested.
As for Unlimited Plus Enhanced subscribers, the new service tweaks are strictly beneficial. Previously, the Plus Enhanced plan's mobile hotspot functionality restricted users to 12GB of high-speed network connectivity before throttling their data speeds to 128kbps. Now, that limit has been increased to 15GB, giving users just a bit more breathing room before they will see a speed decrease.
Finally, Ars reports AT&T will be offering customers subscribed to either plan $15/month in "loyalty credits" to be put towards AT&T's various video services albeit with a couple of restrictions. For example, Unlimited Plus Enhanced subscribers can put the $15 towards DirecTV, DirecTV Now or U-Verse whereas Unlimited Choice Enhanced subscribers are limited to spending it on DirecTV Now.
You can learn more about AT&T's unlimited data plan changes on their official website.