My 6 month old is constantly rolling and moving in her cot. She loves to sleep on her tummy too and is super comfy. But when she stretches up or rolls she hits her head on the side of the cot. This happens at least 7 times a night. What can we do to help prevent head injury? Should we transition her to a floor toddler bed for more room?

Most babies will be rolling over by 4- 5 months. This is very normal as babies grow & become stronger.
Many babies then prefer to sleep on their side or tummy.
This is not something that needs to be stopped. It is important to make her new environment as safe as possible.
https://rednose.org.au/article/what-do-we-do-now-that-a-baby-has-started-to-roll-over

It is not uncommon for babies to roll into the sides of the cot & hit their head or get arms/legs thru the slats.
There is also no evidence to suggest baby will cause any serious injury by accidentally bumping their head on cot sides or putting legs/arms between slats.
“Parents are concerned about the potential for injury from the infant bumping up against the cot or getting an extremity wedged between the slats.
They should be reassured by the Consumer Product Safety Commission(CPSC) data that clearly show that infants suffer essentially no long term injuries from contact with the cot slats, making cot pads unnecessary for infant safety.”(Moon 2007)

Is your baby waking when she rolls to side of cot?
Is she in a sleeping bag (with arms out)?

Transferring to a toddler bed at 6 mths of age may, in fact, create more risks in her environment due to the potential of her being able to access other parts of the house (if she’s not yet mobile, she will be soon.)