I know this has been asked before but wanted to make sure in case things have changed over the years. I have a brand new Deore M592 Shadow 9 Speed rear derailleur and wanted to know if I can replace my old tourney 7 speed rear derailleur with this without having to change my freewheel and chain. I am planning on converting it completely to 9 speed eventually but not right now.
Below 10-speed, specs have very stable and haven't changed over the years (above 10-speed, specs are also stable, but there have been some bifurcation between road and MTB, and between brands). The change has be done in one operation and cannot be done progressively.
No, it's not compatible and it won't work..
Besides the 9 speed Deore have a narrower cage, the biggest culprit is, pull ratios are different.