Here is my rough plan of it. The drawing and part file have the same name which simplifies things.
1. Look for Lowest 6 Digit number in Folder (Store This)
2. Rename the two files that share that number to 000001, Then look for the next number above the originally stored number, rename 00002, continue until all are renamed.
3. Ask user for new start of number range.
4. Step 2, however Renaming all part numbers with the newly entered starting number, add 1, rename next, add 1.
5. Open all the drawing files, cycle through using the Resolve file command using the name of the drawing file, so if a drawing was renamed to 660044.dwg, it will replace the file with 660044.ipt.
Simple enough for sure. The only files contained within the folder will be the ones we want renamed anyway.