Okay so it's a challenge and I'm having trouble planning it mostly due to lack of experience. However it's become a task in my job and I'm doing it mostly solo with an extra hand.
I'm comfortable with Linux and all these servers are Debian or CentOS. Some are VPS, Shared hosts and other are Cloud VMs. I'm new to the company and have only gotten time over the last several monnths to take stock of outstanding issues. They all run plesk or cpanel.
I've found these servers were never configured, just bought and sites thrown up on them with very basic administration in the last two years (disk space clean up and domain/hosting management tasks mostly). There was no iptables set up and no security precautions taken and so these servers are riddled. I figure the best solution here is to start fresh and consolidate our hosting to fewer servers as they had just purchased new shared servers willy nilly. I feel we can support our entire client base on around 1/4 of our cluster.
My problem is that I'm not sure how to plan this move. It will require a LOT of moving sites and DNS changes. I'm thinking it would be wise to seek advice from any wizards out there who deal with this sort of thing regularly or at least have the experience to point me in the right direction.