Some additional direction.

12hr Mode

Display Value to LCD:
  1. Mask off 12/24Hr mode bit
  2. Mask off am/pm bit
  3. Display new masked value with Hex2 modifier

Change Hour Value in RTC:
  1. Mask off 12/24Hr mode bit
  2. Mask off am/pm bit
  3. Convert new masked value from BCD to Decimal
  4. Change Hour value in Decimal
  5. Convert Decimal Hour back to BCD
  6. Set am/pm bit
  7. Set 12/24Hr mode bit
  8. Write new value to RTC Hour register


24hr Mode

Display Value to LCD:
  1. Display value with Hex2 modifier

Change Hour Value in RTC:
  1. Convert value from BCD to Decimal
  2. Change Hour value in Decimal
  3. Convert Decimal Hour back to BCD
  4. Write new value to RTC Hour register


Here is a table that shows two hour values in their various permutations.
SampleTable.txt

Hope this helps.