So i haven't found anything particulary useful about my situation regarding Windows 10 on a 2014 rmbp w upgraded ssd from a 2015 rmbp (SSUBX). If you are having problems installing/running Windows 7/8/10 (any build) then know that you are not alone.
[yes i've used the forum search, google and various other engines]
To be a little more exact:
I have a 2014 13" retina macbook pro - it came with SDNEP (Marvell controller, 2x pciex) 128GB drive.
I dont want to mess with firmware (i use the sleep feature constatly) so i've chosen to upgrade to a genuine Apple ssd - Samsung SSUBX 512GB. Everything went well, macos cloned just fine, speeds were as i'we expected...
...Until one day i've decided to install Windows 10 via BootCamp Assistant - little did i know what a sh*tstorm i've released upon to myself.
First Try: Create USB Windows installer via Bootcamp assistant, boot from created usb drive -> Windows installer fails to start. (tried all builds from 150X - 1909, earlier builds tend to go further in the install process)
Second attempt: Same "bootcamping" as before but i've removed the ssd (the upgraded one) - whoilá the Windows installer starts perfectly but 'course there's nowhere to be installed - i think i got this "" idea from an apple support thread.
Third attempt: ok i realized that this problem has to do something w windows drivers, so i downloaded bootcamp drivers for 2015 rmbp - this gave some progress - i can install windows but after the installer reboots for the first time it stuck on "Setting up services/devices or Please wait w circle animation"
Fourth attempt: Try install windows2go on a pc box - it ran fine on my rmbp w installed ssubx until it didn't. - "Please wait" on windows start screen.
Fifth attempt: Installed windows on an external usb ssd w a neat trick involving using virtualbox for starting the windows install but cut it before the first reboot on install. - This also works - for some time, same as before.
So beware that if you try to install an ssd from a 2015 machine in a 2013-2014 macbook pro it could "cripple" your machine somewhat. (running windows natively)