Sounds like you don't have a SW DA/SA.
If it is new, then your gun is the "0L" type, which is basically a quick action without a decocker for disassembly.
If I'm right, you needn't decock your SW99 at all, it is perfectly safe to carry around with a loaded chamber (think Glock or Springfield XD), no worries, just get a good holster.
-stunks
p.s. reread your post, and it is possible you have a wierdo SW99 variant, but still, it is safe to carry around in SA, but do invest in a quality holster.
p.p.s. if this is the case, then you really just have a SAO, strange. There is NO way to decock this gun, right, then the only way to decock would be pulling the trigger, and with a cartridge in the chamber, that would be a bad idea. Furthermore, the discharge of that round would cause the slide to cycle and load another round, and set the gun again for SAO fire!!! Wierd. Are you sure you don' t have a SW990L?