Apple has extended an offer to give MobileMe subscribers an extra 20GB of iCloud storage for free when they switch over. The free storage was originally set to expire on June 30th, the same day that MobileMe shuts down. After that point, users who’d gotten the fee upgrade would have to pay for the additional storage. Now, though, that free storage sticks around an additional three months, until September 30th.

The change was first noticed over the weekend by Japanese tech blog Mac Otakara. According to an FAQ page about the MobileMe to iCloud transiton, users who have 20GB of MobileMe storage will get an 20GB of free iCloud storage through September 30th. Those who had more than that – 40-60GB – will get 50GB of storage free until the 30th. After that point, standard upgrade fees will apply.

Apple has been pulling out all the stops to get MobileMe users to make the switch to iCloud before they pull the plug at the end of June. Last month they announced a plan to give MobileMe users who were still using OS X Leopard a free upgrade to Snow Leopard as an incentive to upgrade. iCloud is only compatible with OS X Lion, but the upgrade to Lion requires the user to be running Snow Leopard. While Lion only costs $30, Snow Leopard cost over $100. As such, many users simply declined to upgrade when Lion came around. By making Snow Leopard free, Apple effectively reduced the cost of switching from MobileMe to iCloud to $30 (for Leopard users, anyway).

It’s not entirely clear what will happen to MobileMe users’ data when the service is shut down at the end of next month. Most likely it will simply vanish. Apple apparently doesn’t want that to happen, and so is trying to sweeten the upgrade pot as much as possible before then. That being the case, if you’re a MobileMe user who hasn’t switched to iCloud yet, it looks like now’s the time to do it.