Read Me First
Changes from 1.36 to 1.37
Introduction: I realize it's a pain when I change structural details in FlexBar and you have to change your setups. I give a lot of thought to these things before doing it. Please remember I still consider FlexBar to be Alpha Software. Until I am satisfied with the feature set (not until pet buttons at least) and satisfied with a low enough number of known issues will I move it to beta status. Once I do that I will be even more reluctant to change things like these. Until then, these changes were necessary to allow me to keep extending it without working around poor design decisions made early on.
Buttons Above 72: Prior to this, there were 96 buttons, and the buttons above 72 did NOT have a 1-1 mapping of default ID to button number the way buttons 1-72 did. In order to fix this, I have increased the number of buttons to 120 and now there is a 1-1 mapping of default ID to button #. That means that, before you remap anything, button 73 has ID 73, button 88 has ID 88 and so on.
What this means for prior users - The apparent location of your abilities on buttons 73-96 will move. Abilities that were on 85-96 will now be on 109-120. For all but warriors and druids, abilities that were on 73-84 will now be on 97-108. If you had abilities on these buttons and you were just remapping to them, then no change needs to be made as they are still at the same ID. If you were displaying those buttons and not remapping to them, just display the new buttons, drag the abilities to the old buttons and you're done. Finally if you were both displaying those buttons and remapping to those ID's - you will have to move the abilities back to the old buttons, and change and remapping commands to reference the new ID's.
SetTexture: SetTexture used to set the texture by button #. This has changed to setting the texture by ID #. This is to facilitate the new features flexmacro/flexscript/autoitems. While you can still set on events - this ability is mainly for flex-actions (and later pet buttons).
/wait command: Has been changed to /fbwait to avoid replacing /wait voice emote.
Extending FlexBar: The way you hook into WoW events has changed - please read that section.
IsUsable: While this isn't my change, it warrants a word. In interface version 1.4 of WoW, Blizzard change the return value of IsUsableAction. Originally it only indicated that a prerequisite event had happened, (dodge for overpower, parry for riposte etc.). Now it indicates that : A prerequisite event has happened, you are in the correct stance, you have enough mana/rage/energy and it is out of cooldown. If you are a stance dancing warrior who used macros that changed to battle-stance (if you weren't in it) then cast overpower (on the next press) and were used to using isusable to light up/show your buttons when you were not in the correct stance, this will no longer work.
For now, use the new On='TargetCombat' Target='Dodge' to capture this and use a timer to hide the button and an On='CooldownStart' to hide it.
This isn't the definitive answer, but it is the best I have atm I'm afraid.