First off, you'll need a DAT packing/unpacking utility (try Marcelo Bassino's DAT/UNDAT). When you unpack any of the enhanced tracks' DAT files, you'll find MI4 files, M16 files, P16 files, 3DO files, and so on, named accordingly. Most are texture files (in various formats, as identified by their extensions), and are common to both ICR2 and N1 tracks. The Rendition-specific MI4 and M16 texture files found in the ICR2-3D (and N2/N99) DATs are analogous to the standard (non-Rendition) MIP files; similarly, P16 files are analogous to PMP files. What you need to do is unpack the DAT file from the track you want to enhance (e.g., Charlotte) and replace the MIP file for the texture in question (e.g., ASPHALT.MIP) with the corresponding M16 file you want to use from one of the enhanced tracks. Next you'll need to edit the PCK file from the track you're working on (in this case CHRLTTE.PCK) so that the original reference to that texture (ASPHALT.MIP) is replaced with the new reference (ASPHALT.M16). Do this for every texture you've altered. Also, you need to confirm that the number on the second line of the PCK file corresponds to the number of files included; this is a problem when adding sky texture (see below) or otherwise adding additional files rather than simply replacing old ones. Finally, repack the DAT file, rename your original to (for example) CHRLTTE.OLD to avoid overwriting it in case you don't like the changes you've made, and move the newly-repacked DAT file to its track directory. (Note that the other files, including 3DOs, can be manipulated this way as well.)

If you want to add a new horizon and sky texture, copy HORIZ.3DO (replacing the original), HORIZ.M16 (replacing HORIZ.MIP), SKY.MI4 (or SKY.M16), and SKYINDEX.M16. Do not replace the original SKY.3DO; otherwise the untextured sky (as seen from F10 arcade views or in Chase replay views) will appear a strange color. If you want to keep the original horizon but add sky texture (note they might not blend well), add all the above files except for HORIZ.M16 (keep the original HORIZ.MIP). You will still need to add HORIZ.3DO.

And what about those "65" balls in the converted NASCAR tracks? Try grabbing the 65.MI4 file from the ICR2 Michigan's DAT (which, despite its name, is in fact a proper "76" graphic) and replacing the 65.MIP files at those tracks.

How about using textures from N2 DATs? Asphalt or concrete textures appear to work without a problem, though it seems that two files must be imported into the ICR2 DATs for each particular texture (e.g., ASPHALT.MIP and ASPHALT.MI4). I haven't done too much with this, but I've found, for example, that a perfect upgrade for the Cleveland asphalt can be obtained from N2's Talladega, and the asphalt from N2's Sears Point is a significant improvement over that of the N1-converted version. Unfortunately, the ICR2-3D and N2 color palettes differ, so it's a little more complicated a task to use most of the N2 textures. Also note that ICR2-3D's greater color depth allows for more colors and better image quality than is possible in N2.

A word of caution - keep in mind the "base" color of the texture you're replacing; if the base asphalt color (what you see with asphalt texture turned off) is dark grey and you replace the texture with a light one, you'll see the dark pavement "unfolding" in the distance. I've also noticed infield color problems which seem to be side effects of adding new sky textures. One more note: if you're using a texture to replace one with a different name, make sure you change the name of the new texture - for example, when using a texture called WALL1.MI4 to replace one called WALL.MI4.