I had to migrate from Amazon CloudDrive to another cloud storage provider. I don't even know why I signed up for Amazon back then... I have had ~600 GB there so I did not want to re-upload all the data on my home connection (20 MBps).

Use odrive to link your Cloud accounts

I have discovered https://www.odrive.com/ which allows to link multiple Cloud storage providers in a single app, and it allows on-demand download of whatever needed.

I've done this locally on my Mac to ensure things are setup. You get 7 day free trial to allow some nice features like Unsync of unused cloud data.

Connect your cloud accounts

I've connected both my OneDrive and Amazon CloudDrive in odrive.

Spin up an instance on Microsoft Azure

This is the most "tricky" part - you register on Microsoft Azure and you get 170e worth of credit towards their services for first 30 days.

Create a Windows Server B2 virtual machine

I went with a larger B2 Windows Server and created it with 512 GB SSD for storage. This counts towards your 170e credits.

Once the instance is setup, you can download an RDP file and connect to it using Microsoft Remote Desktop.

Setup odrive on your Azure instance

Install odrive on your Azure instance and you should see your linked cloud accounts. Yay!

Make sure that your odrive folder is stored on the large 512 GB SSD (you can do it by right clicking the odrive icon in status bar).

Start downloading/uploading your folders

Choose folders you want to download. Make sure you have 2x the storage available as you:

1. first download them from Amazon CloudDrive. To do that, right click on a folder in odrive folder, check Sync and move the slider to the very right, and click the large Sync button (https://forum.odrive.com/t/how-do-i-move-or-migrate-files-from-one-storage-provider-to-another/106/2)
2. then just wait (you can close the RDP session, copying will be going on)
3. once downloaded (reasonable fast) you just copy the folder to OneDrive folder. That's why you need 2x the amount of storage.
4. this will automatically upload the data using Azure's bandwidth (counting towards your credits)
5. once done, make sure you right click on both OneDrive and Amazon CloudDrive copied folders and select Unsync to free up the space on Azure instance
6. continue with more folders
7. make sure you delete all your Azure resources (VM, storage etc) once you are done

Result

I've migrated 600 GB worth of data in ~4 days without using any of my internet connection at home 👌🏻

The total cost to transfer the data (you pay per GB) and the storage + VM to do the 600 GB was around 55e, all counting towards the 175e credits which is valid for the first 30 days. So in the end it did not cost me anything.