... the status quo!
The ATR-500 will continue in service until being replaced by the -600 verson around 2010/2011.
As for the -900, it remains to be seen if ATR even go ahead with it.
Reasons are; Fuel Economy is just too good for the ATR compared with Q400 - no matter which way you look at it! Plus any merger of the MC and NS pilot groups - for the group common type qual on Q300/Q400 - has apparently ended up in the 'too hard' bin.
As for replacements for the 733? There has been documentation sighted about winglet mods - a-la Southwest 733's. Cheap at approx $2.5m(?) per aircraft for the fuel savings/performance.