After install it you will have win8.1 lumia-cyan version, then go to settings-->update then you will receive a message "A critical update is available" to your windows8 phone,once this update is installed, you will get the lumia Denim firmware update to the version 3051.50009.1451.1004 and win8.1.
Nokia Lumia 625 – Unboxing and first impression; video and photos Nokia Lumia 625 heading to TELUS on October 3rd; will be priced at $300 Nokia Lumia 625 up for preorder in Russia; asking for

This Custom ROM is a build and then capture from Lumia 620 with Windows 10 Mobile Build 10586.36 using Windows Phone Internals developed by Heathcliff74. Only works with this Lumia, so don't test in other models, you can hardbrick your phone.

Currently,Windows 10 Technical Preview currently works with the following phones—as long as they have at least 8 GB of storage, Windows Phone 8.1 was already installed on the phone at the time of purchase, and the version number is 8.10.14219.341 or earlier: Lumia 430. Lumia 435. Lumia 520. Lumia 525.
3.Battery usage overall seems the same as my lumia 640 when it had wp8.1 on it. 4.The general stability is great! Touch is very responsive and there are barely any "loading" screens when an app is launched. . 264 395 100 439 162 254 337 277

lumia 625 upgrade to windows 10