回答“Can I upgrade my Intel i5 2400S to a Intel i7 3770S?”Yes. I did the same upgrade. The CPU is socketed. Took me a good afternoon though.
回答“Is it possible replace CPU from i3 to i5 or i7?”I don't think an i7 will work personally. There's quite a power difference (Max TDP 73 W for the i3-540 and Max TDP 95 W for i7-780 respectively). Secondly it's believed that the Mac somehow utilizes the i-series Clarkdale Integrated Graphics Processor. The i7 mentioned above doesn't have that function.