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While uninstalling old versions of office is technically not required per communication from Microsoft or even recommended by them, you may want to do so anyway and then reboot before installing new versions to avoid files being carried forward from older versions of Office that may or may not be fully compatible. ECI is not responsible for lost files, contacts, etc.. so please back up all work and configurations you wish to save before doing any install.
For Mac: Office 365, 2016, 2019, 2021, etc.. all use the same combined installer that can be downloaded directly from Microsoft, but different Serializers and different Config files. Config files are OS dependent and config files from multiple versions may conflict.
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