This is really simple to do in eagle
On your board, select the component you wish to change and right click it. The option you're looking for is 'Replace'
This will bring up a whole load of selections, in my case I'm changing a resistor so I go to the resistor section and find the package size I want, this example I've changed from an 0402 to and 0603.
The part should have automatically updated without ripping up the tracks.
As you can see in my board though I'm now covering one of my thru holes pads so the odds are you will have to physically move the components also.
Also your track's wont be nice and neatly mitred anymore, but maybe it's just me that's bothered by that.