first you should understand that your device is unsupported for an upgrade, further Microsoft has blocked such devices to get windows 10 builds through insiders . The registry tweak you are using is not reliable and maybe Microsoft has plugged that loophole. Such tweaks could also leave your device bricked and unusable.
To experience windows 10 mobile buy a device running windows 10 out of the box.
Well, any tweaks are violation of warranty. Yep, it's clear the phone isn't, sadly, officially supported by MSFT, so that's why I am writing my questions here and not @ official MSFT forums.
@all other_lumia_620 owners
Somehow I've missed that MSFT does NOT ship updates for RS1 for devices enrolled in Windows Insider program. That's why my phone stuck on build 10.0.10586.494. That was a July's build and on July, 29th IIRC, MSFT stopped Insider Program for RS1.
To get the RS1 build now, in November, 2016, I had to sign-off from Windows Insider program on the phone. When you leave the Windows Insider membership, phone switches to Production Release builds (do not confuse with Insider Release Preview!).
Once I've left the participation in Windows Insider program on my phone, I started to receive the update for 10.0.14393.321 right after checking for updates (didn't even have to restart).
For checkers. If you are stuck on 10.0.10586.494:\
1. Start Windows Insider app and select to leave the Windows Insider program (use ellipsis button at the bottom part of the screen).'
2. Check for updates.
2.1 If checking doesn't show the updates, do steps 3.x
3. Download RootTool.xap from XDA-developers and use the following registry changes.
3.1
HKLM\SYSTEM\Platform\DeviceTargetingInfo
PhoneHardwareVariant -> RM-1027
PhoneManufacturer -> NOKIA
PhoneManufacturerModelName -> RM-1027_1007
PhoneMobileOperatorName -> 000-HK
PhoneModelName -> Lumia 636
PhoneSOCVersion -> 8926
I don't feel changing PhoneMobileOperatorName is so needed, but I agreed to change... Possibly 000-AT is your best Freund.
3.2. Only needed if you can't get RootTool to install. Download Visual Studio 2015 Community Edition and select to install Windows 10 SDK and Windows 8.1 SDK during the setup process; dunno if installing SDKs is necessary, but I needed Windows Application Deployment tool, and I wasn't able to get it with standalone installations of Windows Kits, so I had to install VS 2015 Community Edition.
3.3 Deploy the Root Tool to phone's internal memory.
That's it. I didn't have to use DHT Upgrade Advisor or anything else except for gud ole RootTool app.