Question
As there are production VMs running, is there a way to perform the migration with least downtime?
(All the existing datastores will be presented to the new ESXi hosts)
Yes, that's possible with no VM downtime at all. The general procedure would look like this:
1. Build the new vCenter from scratch (it seems you don't need to retain the old database, which makes things a lot easier) and install the new ESXi 5.5 hosts you want to add.
2. Disconnect the old hosts from the old vCenter and reconnect them to the new vCenter. At the same time or prior to it you can add the new 5.5 hosts already as well.
3. Create the two new cluster objects on the new vCenter with the appropriate EVC baseline and add the respective hosts to them.
4. Upgrade the old hosts to 5.5 via VUM or your preferred upgrade method. You can vMotion VMs around between old and new hosts so there is no downtime involved.
5. Once all hosts have been upgraded to 5.5, you can online-upgrade the datastores to VMFS5.
(6.a) Upgrading VMware Tools and VM's virtual hardware (which will incur downtime) is not really necessary, but you should do that gradually for consistency's sake a while after the upgrade.
(6.b) When you want to decommission the old vCenter is up to you, you could already do it after step 2 is completed or keep it around for some time after everything is done.
The detailed host migration steps depend a bit on whether you use distributed vSwitches. Do you have distributed vSwitches on your existing hosts? If yes, then you may want to temporarily migrate one of the uplinks to a standard vSwitch for migration purposes since the old dvSwitch your hosts are carrying won't be recognized by the new vCenter. This method is described here:
http://virtualkenneth.com/2012/10/19/migrate-to-new-vcenter-server-while-using-dvswitches/
Have a look at the detailed Upgrade Guide as well: