We moved my DD in to a king single bed with a side guard at 18 months, but she never liked the cot and never slept in it as a 4 sided cot (she started in a hammock, then co-slept, then the cot next to our bed with only 3 sides). If your LO is happy in the cot, i would look for another cot for the new bub.

Oh yeah, sore boobs is tough when you have a little person who just wants to get close, and doesn't understand 'gentle' yet.