Problems involved in going from 0805 to (say) 0402 surface mount?
0402 obviously is harder to hand assemble. I am young (&lt;30), moderately near sighted (-4.5) and have a reasonable steady hand. I can hand assemble 0402 boards without magnification, but it is very tiresome to do them for hours on end.

If you need to get them assembled by machine; make sure that:

1) Your assembler's pick&amp;place machine can even handle 0402 at a good rate &amp; yield. Some assemblers may charge more because they are forced to run the boards through one of their more modern machines.

2) Double-sided load. Some boards just need this assembly step, like BGA often puts decoupling caps on the bottom of the board. However some assemblers have trouble with 0402 components on the bottom side of the board. They may force you to keep it at 0603 or greater. The risk is that 0402 components will slip of the board in a 2nd reflow cycle.

3) Electrical properties. Obvious one is of course power rating of resistors. Voltage rating is also likely worse, because of the physical smaller dimensions.
But also consider the voltage bias degradation effect of ceramic class 2 capacitors; for smaller packages in same voltage/capacitance this effect may be worse. 

On the other hand, some properties like lead inductance is lower. In addition sometimes you can put capacitors right at the chip pins for QFP style packages, while routing signals underneath the chip. In effect the loop area for the decoupling capacitor is much much smaller than a larger size capacitor that may be placed further away to comply with signal routing.