I am in a similar situation, my daughter is 23 months now, and I am 6 months pregnant. She will be 2 and 2.5 months when the new baby comes in Feb.
We have been working with her to get her more independant now, so she will feel like a BIG GIRL already, before she is pushed into that role. I have her throw away her dirty diapers in the pail for me, and bring wipes and diapers at changing time, she has to help me put awya her toys before bed, and i have her completely feed herself at meals, she also climbs the stairs (up and down) by herself, since i know I will soon have my hands full, we are practicing this skill now, I just make sure I am a step or two below her, so i can catch her if she falls. I also have her "read me books" and "sing me songs", so maybe when the baby gets here she can do that for the baby too! Everyone I talk to who has done this says letting them HELP with the baby and giving them things to do are crucial!
We did move her way ahead of time (a few months ago)we figured there will be enough changes once the baby is here lets get as many out of the way now as possible. She also has been sitting in a booster Chair at the table for meals, rather than the high-chair since Easter, when she got tall enough to reach. Her nursery was a unisex theme - Noah's Ark, so it will be used for either a boy or girl with no changes. So for her big girl room, we did it all girly, purple with fairies! Once she saw what we were working on and started with the curiosity, we talked to her about it every time she showed an interest - "this is your big-girl room" "this will be your big-girl bed", etc. We made sure it was open for her to play in, as soon as it was child-safe, and after we got the bed up she went in her nursery, and got her lovey-animal, Jeffery, and took him in there herself and showed him the bed and said " big girl bed!" - the next day she moved herself in at naptime - SHE diecided ( I think making it "her" decision to move made a HUGE difference - it went so smooth!)! We did go ahead and buy a second crib - she was doing so well in a crib, she was happy, doesn't try to climb out and sleeps like a dream, so we didn't want to mess up a good thing by switching to a toddler bed too soon and having her up and out of it whenever she wanted. Both of the cribs we have will convert to toddler beds and then to full-size headboards, so they are long-term furniture for those rooms.
If I were you, and had people to care for my child while i was on maternity leave, I would take advantage of it - at least part-time - I think it would help your daughter to keep her routine somewhat the same, for her to still get lots of individual attention form the grandparents, while you get back into the swing of new-born care...Maybe keep her home part of the day - or some of the days of the week, like have them bring her back to you earlier than if you were working, so she can get into the new routine with you too, and bond with you and the baby, BUT - take the help you are offered!!!!
Good Luck - i now the feeling of crossing into that new frontier!
Jessie