In most cases, you subtract 15MHz from the 866, 867 and 868 freqs. All other freqs. do not change. For example, 866.0125 becomes 851.0125, etc.
Older scanners can not be modified to work with re-banded freqs. that have a zero in the 4th decimal place (like 851.2500). Newer scanners need to have custom band plans programmed into them to work with those freqs.
If a scanner has the custom band plans installed, you can simply ADD the new, re-banded freqs. and when they actually change, the scanner will continue to work seamlessly.