I also changed DS's cot-bed into a bed due to back problems, it was excruciating lifting him. He was a lot younger and the transition was smooth, I think because he knew Mummy had difficulty getting him in an out he could more easily accept the change - maybe.
As others have said about night dryness, it comes when they are ready. If you have a gut feeling that she's close to ready it might be worth changing to a bed to make it easier for her to get out of bed on her own for the morning wee, and that could be one reason she might be happier to change from a cot to a bed, if she understands it will make things easier for her, even if that is not now perhaps further down the line when she can call you first thing to lift her out to use the toilet you might use that as a talking point about changing her to a BGB.
You didn't ask about this but I'll mention anyway, I discovered we needed a new mattress for DS, his cot-bed transformed to a BBB but the mattress was only really suitable for a baby, not a toddler. He slept better with a new mattress.