Dilema; when to move lo from cotbed to bed

Lo will be 16mths when no2 is born. She has a cotbed which I intend to use for no2 when they've outgrown the moses basket. We've been looking at something like this for Lyra http://www.johnlewis.com/230178157/Product.aspx (very study, so dont mind spending extra to pass onto siblings).
My dilema is that I dont want to change Lyra's routine or introduce major changes anywhere nr no2s arrival, but I've been informed no2 is on target for being 9lbs plus, so (if they're right, which I know they virtually never are lol) the moses basket wont last that long. I have no idea what the best solution is! I dont want to waste money and dont have alot to spend (will spend extra for something that'll last longer/better quality).

Opinions please!

I'd say that you might want to do it before the baby is born... simply to give Lyra a chance to get used to her new big girl bed, and not see that her cotbed is immediately being used for the baby. But I think that a month or so before would be plenty to allow her to get used to the change so you'd have another 2 months or so?

Both Joshua and Anna were just over 20 months when we took the sides off their cotbed.

Megan is 2.5yrs and we are just about to move her out of the cotbed into a bed bed...but it was just personal preference to leave it this long...We need her cotbed now for Ds who outgrew the moses basket at about 6 weeks old and we put him into the swining crib and now he is outgrowing that...

I would do it before the baby is born as well.
WE had no choice as we needed the cot, Kieran was 14 months when he went into a bed as Anna was born 2 months later and we wanted him to get used to the change before baby came along. Anna was also in a bed at 14 months but that was her that decided that one and then as it just happened Gracie was 14 months as well as she started to launch herself out of her cot. x

I don't have another child arriving but I am no where near ready to consider a bed for Ben who is nearly 22 months, planning on waiting until next summer. So if it were me I would buy another cot for the interim, but then I know I have a wriggly sleeper and he would be getting out of a bed as soon as awake which is not an option as I get a few extra mins in bed when he wakes in the morning and he wouldn't self settle at night.

Thanks all, some good points made. I'm thinking of making her cotbed into a bed in a few weeks just to see how she copes, and if she's fine then we'll buy the bed - if not, we'll wait until baby is born, see how big they are re-assess the situation for there on. Even is baby is only in the basket a few months, Lyra will be older and I suppose a few months makes a big difference in a toddlers ability to do new things. She also a wriggler, so I'm reluctant to move her too soon. She waks frequently in the night, and I'm worried if she can climb out of bed she'll be up for the day then lol! (I need my sleep atm!! lol)
Thanks again, muchly appreciated!!