We process the server migration into 4 steps.
Once the access is given to both the servers (source server and destination server), we migrate all the data from the source server to the destination server without making the new server live immediately.
After all the data is migrated, we'll ask you to test your website's data on the new server, plus we'll point 1 website DNS to the new server to check that things are OK.
Now, when you confirm that all data is OK, we will ask you the time and date to go live with the new server, preferably in the mid-night or on weekends when traffic and load on the server are low.
At that point, we resync files and databases that were modified or recently added after the migration, so no data will lose since the initial migration. Also, we can change the DNS TTL (time to live) for all your domains to 10 minutes for at least 24 hours before going live with your new server. Therefore, it will not be caching your domain DNS for more than 10 minutes and IP changes will be reflected very fast after migration.