I Have a MacBook Pro, with a Core Dual Core processor, and I want to change to a Core 2 Dual, is it possible only change the processor?

The CPU is definatley not a drop in upgrade.  Although, I suppose if you had expert skills in using a surface mount rework station, you'd be able to unsolder the BGA CPU and solder in a new one. 

Theres a company (forgot the name) that does that kind of work but they replace bad CPUs with the same model. But since it's a mac and oddly Apple doesn't cap the firmware on a CPU speed it may work. Unless the chipset isn't compatable. I've seen a mod where some one did this to a mac mini but it's a drop in CPU. Yet it might cost too much. Might be easyer and cheaper to get a diffrent macbook logicboard, unless the logicboard layout is diffrent.

The chips are pin compatible. I don't think the boards are any different, though they do refine logic boards throughout manufacture (original MBP had supposedly had four revisions before I ever saw a single machine here in the UK). But the cost of a Core 2 chip and the fee for 'installation', will most likely be pointless.
The other way you might do it, is to persuade a friendly AASP to order and fit you a Core 2 logic board to your Core Duo.
If I were to do it for someone (not saying I actually would for the record), it would cost more than a new mid-range MacBook, but that doesn't include a discount in exchange for your old (and perfectly good) logic board.
Of course, I cannot guarantee the boards are 100% interchangeable.