Why would you do this?
For one, I'm sure there are many people who have gil over what the new cap will be. So these people will now be forced to store gil on retainers and buy more of them if they want to keep more money?
There are items in the game that already sell for tens of millions. A cap of 99,999,999 is not high enough for people dealing in this kind of stuff.
I'm not one of those people (I'm always broke) but I've played other games where the maximum amount of money proved to be a serious problem.
And -edit- yes, I did mean 100m. If a moderator could please change my topic title to "reducing gil cap from 1b to 100m" I'd really appreciate that.