
DM_Blake |

That would seem to be correct. The secret says "instead of DEX modifier" and it references modifiers, not bonuses, so it even works if your DEX modifier becomes negative (if it only replaced bonuses, then an argument could be made that a DEX penalty might still be applied, but this is not the case). It even says your armor's DEX limit is applied to your CHA instead, so clearly, it's fully replaced and no alterations to your DEX will affect your AC.

David knott 242 |

CMD wouldn't change - and may in fact go up. The -1 from Dex is countered by the +1 from size. I would also interpret that your character would apply Cha to CMD instead of Dex as well (since the majority of the same modifiers apply to both CMD and AC) though I recognize that is not RAW.
The description of Nature's Whispers (Nature mystery revelation) is the main argument that Sidestep Secret should not affect CMD. Nature's Whispers mentions using charisma instead of dexterity for AC and CMD but says nothing about Reflex saves, which means that each of these two abilities lets you use charisma instead of dexterity for two of the three out of AC, CMD, and Reflex saves -- but one of them (CMD for Sidestep Secret, or Reflex saves for Nature's Whispers) is deliberately left out.

hogarth |

The description of Nature's Whispers (Nature mystery revelation) is the main argument that Sidestep Secret should not affect CMD. Nature's Whispers mentions using charisma instead of dexterity for AC and CMD but says nothing about Reflex saves, which means that each of these two abilities lets you use charisma instead of dexterity for two of the three out of AC, CMD, and Reflex saves -- but one of them (CMD for Sidestep Secret, or Reflex saves for Nature's Whispers) is deliberately left out.
I think that's the logic I used for my PC with Sidestep Secret, although it really isn't clear to me that the intent was for Nature's Whisper to be different from Sidestep Secret.