I just built a new computer today with a AMD Ryzen 3 CPU/APU. I put a PCI Express 3.0 x4 (NVMe) Solid State Drive on it and tried to install WINDOWS 7. The USB didn't work and it didn't recognize the drive. So I took the backup SATA SSD out of the old computer to see if I could put a fresh install on it for the new motherboard. Well to my surprise it works with the setup from the old computer, I am using it right now. My question is should I try and install the drivers for the new motherboard on it or would there be conflicts that would cause problems. I am looking at buying a full version of Windows for the new SSD but if I don't have to install it on the old drive I won't.Re-install all drivers for the new rig...Chipset drivers 1st...then all others...and you should be GOOD to GO...
Consider imaging the drive after this works in CASE you wanna move to a new SSDIf it works in your old SSD then you can definitely install all the drivers in it and if you want to move the OS to the next SSD, as told by Patio above image the drive into the new one. Hope it works that way and you can work with the new SSD.The native Windows 7, on the other hand, does not recognize or work with PCIe NVMe SSDs. Nor does it come with in-box drivers. If you wish to install the Windows 7 system on a PCIe NVMe SSD, PLEASE refer to the link below, or consult Microsoft for information.

https://support.microsoft.com/en-us/help/2990941/

So you will need to install the 2 updates to the Windows 7 on the old SSD and then find a program which will see the NVMe SSD to do a copy. This link contains a link to software and a tutorial.

If the motherboard is new it would pay to check there is drivers available for Windows 7.

If the computer is not booting from USB it is likely a bios setting, most bios's have a key to press to enter the boot menu normally f12.