I moved DD#1 into a big bed at 27 months...when DD#2 was abotu to be born. We had no probs, but she was alittle bit older.
Maybe do day sleeps in the 'big bed' and keep the night sleeps for her cot....let her get used to the idea. Get her to help 'decorate' her new room, make a fuss over the new bed, and what a big girl she is etc etc. .
I will be enduring the same battle come sept. ALthough my DD#2 will be 19 months...but i think the above is how i would do it.
Good luck.