I had to call Mint Mobile to deal with an account issue today (free 3 months not applied from one of the plans I purchased in December). Before I got to talk to anyone, I heard an announcement that existing plans were having their data allotments upgraded without additional cost with everyone's next cycle. Indeed, the new allotments are shown on Mint's web page.
$15: 3gb --> 4gb
$20: 8gb --> 10gb
$25: 12gb --> 15gb