Hello Coctien12, I migrated my own phpBB forum to XenForo and I moved the vBulletin 4 forum of NationaalComputerForum from an English host to a Dutch host en then migrated vBulletin 4 to XenForo.
First it is in my view necessary, when you start the process to close your existing forum for the time beïng.
You make an announcement to tell your members that everything will better en more beautiful afterwards.
On the server you have made a subdomain with a simple name, for example mybb.your domain name.
Then copy all your mybb forum parts to that new domain.
That done, you clear everything in the main folder en you can upload the install files of Xenforo 2.
Additionaly you also need to make a new database for XenForo - take care to record everything, because you need these for installing XenForo.
Is XenForo up and running then you mast take care of this: in Admin click on Setup > Options >> Board Active > make your forum inactive.
Do not worry - you will see everything but none of your members can log in.