Will the migration cause downtimes, and if so for how long?

Depending on the type of server you have with Contabo, you may experience downtimes. 

If you have a dedicated server, there will be downtime, but it will be scheduled overnight and will last a maximum of 14 hours. You will be notified 3 weeks in advance, be sent a reminder just before the migration and receive confirmation once the process is complete.

If you have a VPS, there will only be a very small downtime. VPS migrations are performed online and if any downtime is noticeable, it would last less than 1 minute.