It's possible to flashme a DSlite with flashme v6, somebody has done it already at SCdev forums , and it works.
However you loose the function to adjust the brightness, but if you set it to your likeness before you change the firmware, it's gonna stay that way even after flashme.
The old DS could never get the same level of brightness of the Lite because it's an antirely different kind of display.
I't umproved in many ways, like more vibrant colors, more contrast and less glare.In fact, it's not even possible to switch the backlite off because the screen does not reflect any lite at all, there fore you can only switch it to the lowest level of brightness.
Even if you don't change the brightness, the DS lite screens are a big deal so much better than the DS's.
All you need is a passme2 device or such.
I am sure though a new version of flashme is well on the way, Loopy seems to have his hands on the new firmware already....