Can I convert from a 52/42/30 crankset to 50/39/30?
When you do drive train swaps, you need to check the chain length, and the capacities and limits stated on the front and rear derailleur (look at their data sheets). 

1) Check the data sheet for your front derailleur. It will specify the sizes of the chainrings and the teeth differences required. Almost surely it will be fine though. You'll have to fidget with the clamp position a bit. 

2) You'll end up shortening the chain by a few links, ideally (you can run a too long chain safely, but why bother?). The cassette doesn't need to change. 

3) No - so long as you stay with a &lt;=9 speed Shimano RD and Shimano Road FD (I believe &lt;=10 speed), you're OK. The RD doesn't need to change either. 

Aside: 52/42/30 cranksets aren't uncommon.