First thing you should know about managing Exadata VMs is that you can migrate Bare Metal Oracle cluster to OVM cluster. Conversion of Bear Metal to OVM can be achieve with 0 down time or minimum downtime, depend on various migration methods.
Memory: You can decrease or increase amount of memory allocation to user domain with proper planning. For example: if you want to decrease memory allocated to user domain , you should consider instance memory parameter and make sure you still have enough memory left for user domain to support SGA/PGA for all the running databases. Memory changes to user domain is not dynamic and will require restart of user domain.
CPU: Similarly, you can increase and decrease number of vCPU’s assign to a user domain. You can also dynamically increase or decrease number of vCPU’s assign to user domain as long as it will exceed number of vCPU’s assign to that domain. Overprovisioning is possible but not recommended, it will require full understanding of workload on all the user domains.
Storage: Like CPU and memory you can also increase size of underline storage for any user domains. You can add new logical volume, increase size of root file system and increase size of Oracle Grid or RDBMS files system. . You can even add a new mount, if you like to add another Oracle RDBMS home
Backup: With addition to all other backups you need to backup both management and user domain in virtualized Exadata environment. As a best practice backup destination should reside outside of the local machine.