Gut feeling is that this would be more of a sideways move.
The spec sheets for both aren't directly comparable, so it's not a simple job to see the differences. On the plus side the newer amp has digital inputs (coax and optical) and a 192kHz compatible DAC to go with them. They're quoting a higher Signal-to-Noise ratio too for a High Level signal, but that could be just a technical difference in the way they're making measurement.
The amplified power output is quoted as the same 45W/ch, but - as long as it's not a typo - on the older amp they're quoting a much lower THD figure (0.0008%) compared to 0.08% for the later model. Without looking in to it more deeply the numbers on their own don't mean much because there's no reference to how they were measured: Was it single channel or both driven, was it 1kHz or 20Hz~20kHz, and was it peak or RMS/continuous, are they measuring on to 6 Ohms or 8 Ohms. You'd probably need to download the manuals for each which would then give a better idea if it's a level playing field.
The bottom line is probably about whether the digital inputs and DAC are worthwhile for you. The 6003 was a good amp when launched, and will still be equally competitive in sonic terms today. If you need a DAC, and if the cost of the amp change is less than buying an external DAC and cables (an you have to factor in an extra box plus controlling it and an extra power socket) then, so long as you're not dropping any wattage, go for the new amp. If it's just changing for changing's sake though then I'd be tempted to look at something that would be a definite sonic step up.
