Downtime won't necessarily be required for the VMs if he uses a compatible EVC mode on his HP blades. How old are the IBM blades? are they intel processors?
We just did this in our environment, going from HP BL460c G6 (ESX 4.0) blades to Cisco UCS B200 (ESXi 5.1) blades. In our case, we set the new UCS hosts to run in "Intel Nehalem Generation" EVC mode, and this allowed us to live migrate about 80 VMs from the HP hosts. Nobody even realized it happened, as the VMs will only drop a single ping, as with any other vmotion.
What will require downtime is the eventual vmtools and vm hardware upgrade that you'll want to do. It can wait for your next maintenance window, though.
I'd recommend building a new vCenter from scratch well in advance so you can take your time setting it up. Just use a trial license, it is good for 60 days. Once you are ready, you can convert your old vCenter 4 license to a vCenter 5 license and apply it to your new vCenter, replacing the trial key. Your old vCenter will continue to work, but you are technically unlicensed from that point on the old one.
We built our new vCenter as a VM on the new UCS blades. We ran into a "chicken & egg" issue setting the EVC mode since it is a cluster-level operation, and you need vCenter to do that. however, you cant change EVC level with powered-on VMs in that cluster! To get around that, we made a temporary cluster with a single host, moved new vcenter to it, made our change to the other cluster, and then moved it back.
Remember to configure your vswitch and datastores exactly the same in the new environment so you don't have problems during migration.
We used a kind of "life-boat" method to move the VMs from old environment to new. In the old environment, I disabled DRS and filled up a host with the VMs I wanted to migrate. Then, I disconnected the host from the old vCenter. In the new vCenter, I added that host into the EVC-moded cluster, and then migrated them off. They only dropped one ping. Once the life-boat host was empty, disconnect from new vCenter, reattach to old vCenter, and repeat until your old environment is empty.