In our experience, smaller colo customers (1 - 5 servers in our experience) are not a challenge to migrate to cloud. Their requirements are low and the cost benefit short and long term for them is easy to show. The flexibility alone is the ideal selling point.
However, once we get into the 10+ servers, we start to get questions around performance and custom configurations around VPN, Firewalls and private links.
So this then comes down to how is your cloud setup, what features does it have, how easy are they to use and enable.
However we find that performance is still an important point. With the introduction of solutions like Solidfire, performance in the cloud is becoming a challenge that can be overcome. There are a couple of other storage providers also starting to focus heavily on guaranteeing performance.
I would note that Techy is right, these conversations generally only happen once a customer is the end of their contract with the colo provider, or at the end of their hardware's lifecycle.