DD was 23mo. I moved her as DS was due when DD was 2y 1m and I didn't want her to feel he'd come along and stolen her cot. She took to it really well, we put the bed up in her room with the cot still there, and took the cot away after about a week.
thank you for your responses. My daughter is not long 2 and sleeps and settles ok at the moment in her cot. Her cot doubles as a junior bed so might take one side off at the weekend and see how she goes.