It would be between a "down" grade and a upgrade.
But it depends what you find important.
Processor wise they are about the same.
The 640 has a larger display but the 925 has more ppi than the 640.
Camera wise the 925 is "better" but the 640 has a "better" selfie (front) camera
Battery wise the 640xl is better.
The 640xl has less internal memory but SD support.
The 925 can be charged with QI but the 640xl not.
(But I think with the 640XL you may get a year long office 365 for free)
Microsoft Lumia 640 XL vs Nokia Lumia 925 - Phone specs comparison
If you find battery live and size important but camera and ppi not then the 640 is an upgrade.
The LG has indeed an impressive spec.
If you buy the LG and only use it for about a year ( or less) till a windows phone flagship comes is I think a waste of money.