Microsoft has stated that all current Windows 8 hardware will be able to upgrade to 8.1. I would not be surprised though if the lower end models (for nokia 720 down) will see some noticable performance impact similar to what we saw when WP7.8 came to the Lumia 800 and 710 (which are noticably less fluid and snappy on 7.8).
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Yes, the Lumia 520 and 720 will also get the WP8.1 update.
"All of this will come to your Lumia smartphone running on Windows Phone 8 in an update package called Lumia Cyan-starting this summer."
So, that is 3-6 months from now. Or at least before Christmas. If all goes well..!