Hi Phillippe,
BI Contents are downward compatible. Higher version of BI content can work with lower version of ECC system.
"PlugIn" and "PlugIn Basis" are downward compatible. It means: e.g. an SAP Application like ERP with a lower PlugIn/PlugIn Basis release can replicate data into a SAP BW Server which has a higher PlugIn release AND vice versa - an SAP Application with higher PlugIn/PlugIn Basis release can replicate data into a SAP BW Server which has a lower PlugIn release.
Please refer 153967 - BI Content Release Strategy
Thanks,
Shakthi Raj Natarajan