I switched my son to a toddler bed at about this age. The signs for me were: Strong enough to climb out of the crib (only a matter of time), and independence to do things on his own. Our crib has a toddler rail, so first we used that as a transition. Warning: we had to restructure everything in his room now he had free reign, and nap times were more difficult as first (too much excitement for having access to his room). Then I purchased a new bedroom set, a twin bed with a trundle. So I transitioned him to his twin size bed IN the trundle near the floor first. I left the crib with the toddler rail so that he could choose. It became very evident that there was no turning back, and my son prefered the twin bed (so I disassembled and removed crib). After a month or so, I assembled the rest of the bed frame and installed the safety rails. I also purchased a new comforter from Bed Bath & Beyond that has microfiber on one side and really soft cuddly fabric on the other (similar to his existing crib size blankets). The comforter is perfect! He sleeps through the night and does NOT get repeatedly in and out of bed. I must say that he sleeps better in his bed than in the trundle, as I think sleeping lower to the floor was too much distraction. However, it gave me confidence to know that he would not roll over onto the floor, and as we did not use a top sheet, it slept like in the crib. I recommend that you watch the signs for readiness. Some children move to beds sooner than others. However, definitely do not want to move too soon. Also, be prepared for some bumps in the road during transition time. At one point we reassembled the crib for nap time, and determined my son really didn't care where he slept...If he didn't want to sleep, then he wasn't going to sleep! Once he figured out how to open doors, we added a childproof door knob cover on the inside of his door.
Some children sleep better once out of the crib (not so confined). Others like the security. My recommendation is to not rush it. If your daughter is not ready, your new baby can sleep in a basinet for the time being. Also, once the baby is born, it may serve as a nice transition for your daugher to move on to a "big girl" bed so the "baby" can have the crib. At this point, you can make a big deal about it (but I would wait until your daughter adjusts to the new family member). Good luck.
Regarding waking during the night, we were always very aware of his types of cries. If it was blood curdling, then we went into his room right away. Once his foot was stuck in the rails of the crib! If it was light crying, we would leave him alone. If he cried more that 5-10 minutes or it got progressively worse so that he was so worked up and would never go back to sleep on his own, then we would go into his room - without picking him up or turning on the light - and tuck him back in. We also have a lullaby CD that we play each night as he falls asleep as a part of the routine. So if he if woke during the night and we had to go into his room, we would turn on the CD again. Best wishes.