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..!
If i update my Windows Phone Nokia Lumia 720 will my warrenty goes?
Can i update from this link http://www.windowsphone.com/en-in/store/app/lumia-
Any new updated in 8.1 in future will i get it afer upgrading this?
That link is for an app which claims it will show you the new features coming with WP8.1 - it will NOT update your handset to WP8.1.
Your warranty will not go if you update to the official WP8.1/Lumia Cyan version for your handset. You can check when it will be available for you HERE