change block tracking

Can we keep vm sanpshot while we enable CBT on VM or We have to delete the snapshot before we enable the CBT?

You can keep the snapshot, the only change to the VM, by enabling CBT, is a change the VMX file.

I was told that I have to upgrade the vm hardware fom ver 4 to 7 this case ,I am going to take the snapshot before we do anything,

Correct, Change Block Tracking is a feature only supported in virtual machine version 7.0 of later.

You could just make a copy of the VMX file!

The line in the VMX is ctkEnabled = "TRUE"

each disk will also have a

scsix:x.ctkEnabled = "TRUE"

I am not sure whether upgrading the hardware ver will enable the CBT by default, if not then I have to enable manually.

No, it does not enable by default, it needs to be enabled. Most good backup products will make the change to the VM to enable CBT.

see here
VMware KB: Changed Block Tracking (CBT) on virtual machines

VMwarew KB: Enabling Changed Block Tracking (CBT) on virtual machines

Andrew,
If I understand correctly.
leave the snapshot on mahine.
shutdown the VM
upgrade the hadrware
shut down the VM again
edit, options -advance option .
no need to delete vm while I am doing all the above steps.

There is a step missing, before you Upgrade the Virtual Hardware, you need to Upgrade VMware Tools (unless already done!),and this requires a reboot.

So your steps are correct, other than VMware Tools upgrade if required. (must be done before Upgrade Hardware).

I have an appliance whic is centoOS based, I beleive that I can not upgrade the vmtool unless the softwre vendor upgrades it,
Can I still upgrade the hardware ver and enable CBT?