Usb 3.0 motherboard usb 2.0 case
I have a usb 3.0 motherboard but my case has usb 2.0 will it work I don't mind if it will have the speed of a usb 2.0
They are compatible. Just use adapter like this. (if case do not have one on)
Most boards still have 10pin[2x5](8 actually used for 2 ports 1 is a place holder and a missing one to prevent reverse plugging.) usb 2.0 plugs anyway.
As long as your board has that you will not even need an adapter.
If you had a case with only usb 3.0 the adapter above would allow the 2.0 ports to use the 3.0 plugs on the case.