Reddit reviews Nokia Lumia 830 GSM Smartphone, Black - AT&T - No Warranty

I would say it wouldn't matter, since all current Lumias were already confirmed to get Win10. However, since you're looking at the 830 you don't seem to be particularly interested in a flagship (unless you don't like the size of the 1520 and also don't like Verizon/their failure to update so you don't want the 930). I would assume the only MAJOR releases as far as phones go for next year would be successors to the 1520, 1020, and the 930(maybe), which would all be flagships. Also, new phones next year will cost a bunch, probably around 200 dollars on contract, a price that will drop over time (if you get a 2 year contract on the 830 then by the time the contract is up you can probably get the flagships for 50 dollars on contract). I bought the 1520 and went through this same thought process (I bought my 1520 2 weeks ago), so I can share you my reasoning when it comes to "waiting for Win10"

When buying a Windows Phone, consider this:

• Windows Phones will ALWAYS work on really low end hardware, including Win10 for Phones, as all Lumias will get the update (including the really low end ones like the 530 and 535)
  
  
• Win10 for Phone is GUARENTEED for all Lumias, MS tweet for proof. Here is a Screencap of it if you can't look at Twitter right now (stop browsing reddit and get back to work :P)
  
  
• If your current device isn't cutting it, updating will be entirely worth if for the year you use it until a new flagship comes out
  
  
• Flagships (even Windows Flagships) will cost a lot of money on release, so waiting will bring down the price.
  
  
• SensorCore - A feature that Microsoft does a poor job at marketing, but it will enable you to use the "Hey Cortana" feature of Lumia Denim, checking what phones have SensorCore can help you in the decision (625 doesn't have it, 830 has it), so if that's important to you, then you can get that feature for the 830 when Denim is rolled out, but you can't do that on the 625.
  
  
• Glance Screen - Another Lumia feature only on some phones, requires a specific hardware part as far as I understand it, so if your phone has it is entirely the deciding factor of if you can get it. (830 has it, I believe the 625 does not.)
  
  
• Hardware - Overall, you need to consider the hardware jump. While WP8.1 runs well on all devices, "Resuming..." is a big problem for 512 MB RAM devices, the 830 has 1 GB, so you can expect a drop in that. Also, the 830 has a 4 core processor, the 625 has a dual core. 830's battery is also removable, so if that appeals to you, then there.
  
  
• Flagship VS Mid-tier VS Low-end - Windows Phone 8.1 runs well on all of them, we can expect Win10 for Phones to work the same way.
  
  Also, the whole "what if I wait a year" aspect of this will always get you in trouble. A new phone will come out the next year all the time (maybe not a successor to your device, but one that might appeal to you more). Stick with the "my situation now" thought process. If you try to live in the future with phone's you'll always be waiting.
  
  TL;DR: I wouldn't expect a major non-flagship phone next year, the prices of the phones released will be high, and they will be released late into the year most likely, etc. If you don't like the 625 / want more power, go for the 830, but if you don't care about additional features and speed and want a flagship that isn't the 1520 or 930 and you're fine with your 625, then stick it out. Maybe consider buying the 830 outright instead of on contract if you really want the 830 and a flagship from next year, of course not a lot of people can just drop a couple hundred dollars on a phone like that (it is 400 dollars on Amazon (300 dollars off) right now if that interests you.)
  
  edit: formatting

Lumia 930

Lumia 830