You have to use the 2.66GHz logic board, the 2.53 has a different shape. Take a look at the differences between the two.

2.53GHz logic board - MacBook Pro 15" Unibody (Mid 2009) 2.53 GHz Logic Board

2.66GHz logic board - MacBook Pro 15" Unibody (Mid 2009) 2.66 GHz Logic Board

It will but you'll need the center bracket mount piece (the one in the 2.66 and up are different, about $20 more less) that the fan and the logic board mounts to in the case, also you have to get the correct heat sink (meaning you can't use the one from the 2.66), better get a logicboard with the heat sink still attached and un disturbed, that way you don't have to clean and reapply the thermo coumpound. you will need one extra logic board screw, because the 2.53 one uses one more screw to mount then the 2.66 and up do. and the logicbaord would need the mid 2009 2.53 (single fan) size fan (the fan is bigger then the 2.66), But it all works if you want to swap. I did it, upgrade and downgrade. i upgraded a 2.53 to a 3.06, and i replaced a 2.8 with a 2.53 So I know all about it. Personally i would upgrade not downgrade. the 2.53 logic board, i find are a little buggy. Why not try for a used 3.06 gHZ logic board, then all you need is just the logic board (preferably with the heat sink attach, otherwise you need thermo compound). They dont cost a whole lot used, i say a ball park range around $400-$700 used on ebay when you find one.

Also you would need to change the keyboard and trackpad as the 2.53ghz models are not compatible with the 2.66Ghz + models.

That is incorrect, the keyboards track pads are compatible. I have done this many times