migrate or move Zimbra 8.6 on RedHat Linux 6.6 to Zimbra 8.8

Ubuntu is just as easy to use for Zimbra, but if you're used to Redhat, then you should go with CentOS, since it's more familiar.

I prefer CentOS. can i install a fresh zimbra 8.8 installation & restore data from old server?

Yes you can. I highly recommend zextras migration tool. Its free

https://www.zextras.com/suite/migration-tool/

I had a quick review of zextras migration tool. do you have been used it earlier?  could you please give me a brief about it?
thanks,

Yes, I have used it for almost every migration I've done. One migration was over 1000 mailboxes spread over a multi-server setup and it worked quite nicely.

Basically once installed on the old-server, you run it and it creates a backup file that includes all the zimbra data and settings. This means not just user mailbox with all the items, but domain/server settings as well. You have the flexibility to export single domains in multi-domain setup or the entire server.

On the new server. You install z-extras backup trial. The trial version lets you import a zextras migration files as long as its not older than 30days. The full version lets you import zextras migration file of any age. Once installed, point to the file from the old server and once again you can pick and choose what you wish to import from single mailboxes, to entire domains, to the entire server.

Here is a link to the migration tool guide on their website. Although the process is very easy, I recommend going through the guide before doing anything on the server.

https://wiki.zextras.com/wiki/ZeXtras_Migration_Tool_Guide

Migration Checklist
To efficently migrate data from a Zimbra server to another you will need:

• -- Full access to both Zimbra servers (admin access to Zimbra Administration Console and root access to the machine)
• -- ZeXtras Migration Tool installed on the SOURCE machine
• -- ZeXtras Suite, with the ZxBackup module activated, on the DESTINATION machine. Both Paid License and Trial Mode will do.
• -- A "ferry" store to move the exported data to the new server, approx. the size of the /opt/zimbra/store/ folder on the Source machine. This can be a network share o a dedicated device in a physical environment or a virtual disk on a virtual environment. Mind that during the migration you'll need to move the "ferry" store from a machine to another.

After the migration completes can i uninstall zextras from zimbra server. Am i correct? Then no any expiration on new server and no need to stay zextras.
Thanks for your valuable input. Greatly apreciated.

That is correct after the migration is complete you can uninstall zextras from the new server.

I enabled proxy & memcachd on a test vm & upgraded directly to 8.8 version and it was smooth.
thanks.

I also upgraded my production vm to zimbra 8.8.9. but my base OS was RedHat 6.6. I converted it to CentOS. now I want to upgrade to CentOS 7. As I found there is no way to upgrade to Cent OS 7. I tried zextra migration tool & it says that it does not support latest zimbra 8.8.9 version. now what can I do? is there any possibility to backup & restore zimbra 8.8.9 from CentOS 6.6 to a fresh CentOS 7 ?