May be the second most popular lumia in the world but it receives updates late and still doesn't have a TRUE successor. (625 & 630 don't really improve much in my opinion).
8.1 update is by Microsoft and you can easily get the update without carriers approval via developer preview program , Lumia 620 got the black firmware late , blame nokia for this black update is made by nokia , Microsoft did their job by sending early access to OS update kicking carrier approval.
Dude Lumia 630 has not been released yet. What if that phone is actually a 520 successor.
Anyways back to your question, Lumia 620 will get updated to 8.1 without much waiting. I suppose if not on 2nd April but definitely before 10th.
Microsoft has their awesome awesome Developer Preview Program. No more waiting.
I fully agree with "still doesn't have a TRUE successor". The rumoured Lumia 630 lacks flash. LOL what a pathetic decision by Nokia!
If only Nokia could release a 4/4.3 inch Lumia 620ish phone with 1 GB RAM, capacitive buttons intact, Snapdragon 400 and Windows Phone 8.1...