Fun, cool announcement to make.
PowerStreamECM, a tool I have built for large scale ECM legacy migrations now supports EXTREME Mode for Office 365!
Extreme mode takes advantage of the Azure to O365 path using the content deployment API methodologies of SharePoint. The fun interesting thing about Extreme mode is its designed to work with hard drive based deployments to Azure. Sending 1GB, 10GB, 100GB to Azure is ok...try sending 30TB, or even 100TB to Azure over standard network lines....it just doesn't work. Extreme mode allows you to partition your data across multiple drives and then send those drives to Azure for upload. The entire process takes about 2-4 weeks.
Once the data is loaded, you can then fire a secondary process to upload and modify any custom properties based on custom business rules to update the pre-loaded binaries.
Once uploaded, custom and 3rd party tools can apply the retention rules you are looking for to destroy old records or do eDiscovery. Unfortunately, the only thing about uploading 30-100TB of data is that O365 can't handle it well for certain features. That's where the experience of Hyper-scale design comes into play. We have built a set of rules and software that allows you to do hyper-scale O365 deployments easily (although, time always plays a factor in hyperscale, just look at big data)!
If you have an old ECM system that has over 30TB of data that you want to move to O365, give me a shout. I have seen it all at this point and can tell you what works and what doesn't!