Numbering Exceptions

[Patton]

If more than one choice exists for a numbering exception(s), what determines the choice and can that subsequently change if a shift occurs?

Here's the situtation: K lines up in scrimmage kick formation on 4th and 2 at the 50. The following numbers are all on the LOS and 30 is the snapper:

88 | 18 | 72 | 30 | 57 | 63 | 45 | 80

Although 18 appears the most logical choice as the numbering exception, 45 could be as well. What if 88 shifts to the backfield, does 18 become eliglible since 45 could have been the originally intended numbering exception?

What if 80 shifted to the backfield, do we allow 45 to become eligible?