The original DS games will not be able to take advantage of the slide pad because they were not programmed for it. Just like how some PSX classics on the PSN store like FF7 cannot have full analog control for movement. You can still use the analog stick in replace of some of the buttons but you will not be able to use it for 360 degree movement.
Like others have said, there is probably little chance of the analog stick being analog. There is the possibility of the analog stick being used as a replacement for the D-pad, but it would still just be replacing a D-pad, and as such, not allow for full analog control.
plus not all the games use the d-pad anyway so in those the slide pad would not be able to be used. --- Today I step into the shoes of a great man, a man by the name of Dwayne Elizondo Mountain Dew Herbert Camacho.