You need to upgrade your SD card for WP8.1 and some bad news for 520 owners
Windows Phone 8.1 brings the new capability to install apps on the microSD card, if developers allow of course, and will allow users to no longer be limited in any way by the built-in storage on their phone.
According to them a minimum of Class 6 SDHC card is required, with Class 10 recommended.
Class 4 and below will not be supported, with the OS automatically detecting the speed on insertion.
If the card is too slow users will not be offered to move apps from the phone to the card.
The requirement should also help users avoid performance issues when saving video to external cards, which can be stuttery when bitrates are high.
Separately it also confirm that the 3 column screen option will be confined to screens with a physical size bigger or equal to 4.5 inches, excluding the Nokia Lumia 520 and 620 for example.