Hi Neil,
As per SAP note 129352 - Homogeneous system copy with SAP MaxDB
You can copy 7.5 MaxDB database to MaxDB 7.9
As you have content server on MAxDB refer to below SAP notes for guidelines on Content server system copy
962019 - System Copy of SAP MaxDB Content Server Database
1619726 - FAQ: SAP MaxDB Content Server
Hope this helps.
Regards,
Deepak Kori