Yes. Yes, it is perfectly fine to switch from ready-to-feed to powdered formula since they have the same nutrition. I do have some patients who prefer one more than the other or have problems with spitting up more with one than the other. If you look at the two, you will also see that they look different with the ready feed (enfamil lipil at least) looking darker and thicker than the powder.
Yes. If your water source is safe to drink, it is perfectly ok to switch - and alot cheaper for you too. If breast feeding just hasn't worked out for you and you use formula, just make sure you mix according to directions. You don't want to cheat your baby out of the nutients she needs for optimal growth by mixing wrong. And remember not to shake hard before feeding - you will end up with gas!
Yes. Powdered formula or ready-to-feed liquid formula are both fine to use. A baby can switch back and forth to either formula, at any time.