The question is simple. I have a cluster with 2 ESXi 4.1 and I want to move to 5.5 version. It's a good idea upgrade directly? Or better do a fresh install of 5.5 version directly?
In case I could upgrade, is better doing with Update Manager, o manually with USB boot image?
When moving across such a large version chasm, I always try and do a fresh install. If you're using Enterprise Plus, use of host profiles makes that process quicker as you can template a given host and quickly bring others back up to those configuration standards.
No, it's Essentials Plus.
I've just read that if you do an upgrade, the partition table remains in MBR not GPT. Then the LUNs will be of 2 TB max. Is it true?
I assume that your vCenter is already running version 5.5 and that your hardware is supported by the vSphere ESXi 5.5, right? Anyway, since your host maybe is running for many years, I would consider a fresh install instead of upgrade, this way a lot of possible "garbage" from the old installation will be removed and you will have a clean 5.5 installation. But, you will need to reconfigure all hosts settings from scratch as well. From VMware documentation, you can upgrade directly from ESXi 4.1 to 5.5 without problem and that is fully supported, see: VMware Product Interoperability Matrices
After the upgrade if you upgrade the VMFS 3 to 5, the partition table will continue to use MBR, BUT it will change automatically to GPT if you expand the datastore to above 2TB, see: Upgraded VMFS-5: Automatic Partition Format Change - VMware vSphere Blog
No, ESXi's and vCenter are 4.1 version both. Then no discussion, is better a fresh install?
On the other hand. All my VM are in a disk fiber cabinect. If I unplug it before the installation and when everything is running, I plug it again, I will be able to import the VM to the environment?
My VMs will be compatible with 5.5 or 6.5 in the future?
try to use vSphere Update Manager to Perform Orchestrated Host Upgrades.
Orchestrated upgrades allow you to upgrade the objects in your vSphere inventory in a two-step process: host upgrades, followed by virtual machine upgrades. You can configure the process at the cluster level to automate more of the process, or you can configure it at the individual host or virtual machine level for granular control.
For example, you can define a host upgrade baseline to upgrade an ESXi 4.x host to ESXi 5.x, or you can define a virtual machine upgrade baseline to upgrade the VMware Tools and the virtual machine hardware to the latest version. Use wizard-based workflows to first schedule host upgrades for an entire cluster and then schedule a virtual machine upgrade for all the virtual machines.
You cannot use Update Manager to upgrade a host to ESXi 5.x if the host was previously upgraded from ESX 3.x to ESX 4.x. Such hosts do not have sufficient free space in the /boot partition to support the Update Manager upgrade process. This problem also affects some 4.x ESX hosts, even if they were not previously upgraded from ESX 3.x. Hosts must have more than 350MB of free space in the /boot partition to support the Update Manager upgrade process. If the host that you are upgrading does not have more than 350MB of free space in the /boot partition, use a scripted or interactive upgrade instead.