Why? A few reasons.
1. The CPU compatibility list is from the motherboard maker.
2. If you do swap in a compatible CPU your OS install such as Windows often fails and sometimes invalidates your license. License issues are between you and those that supply such. Dell or Microsoft in this case.
3. If you swap out the motherboard the license for W10 and from Dell is invalid except for a very specific reason and as such you must destroy the old motherboard. There are discussions about that so I end here.

There is usually NO PERFORMANCE INCREASE when we change motherboards and the 1200 isn't much of a slouch so I can't guess why that's on the change list.

1. Did you expunge all HDDs from the system in favor of SSD?
2. Yes, upgrade the GPU.