I don't have important data on the device. Are there some partitions I should keep? If no, will I be able to revert back to Windows and possibly reactivate my copy if I change my mind in the future?
Thanks in advance :)
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If you want to keep Windows as dual boot, just keep the Windows partition, and install Ubuntu in another partition.
If you don't want to keep Windows, you can format that partition during Ubuntu installation. Later on, you can still install Windows (which will delete the Ubuntu bootloader), and activate it online (previously, you would need a product key to activate Windows, but in the recent years, Windows recognizes your hardware and automatically activates).
To directly address the two questions:
Are there some partitions I should keep?
There's nothing you must keep, but having said that keeping certain partitions may have an effect on the overall experience. See #2 and #3 for some thoughts on what can be kept and the results of doing so.
If no, will I be able to revert back to Windows and possibly reactivate my copy if I change my mind in the future?
If you do remove the Windows boot partition, options #1 and #2 provide routes to reinstall if needed. As far as activation, as others have noted all modern laptops have the license baked into them, presumably in the BIOS somewhere, so all you need to do is install Windows and it'll just work.
That said, in my humble opinion, there is no best single solution. I can see several possible courses of action, each one has various benefits and liabilities.
I would suggest when creating the Ubuntu Installation not overwriting the Windows Partitions. Main reason being if you want to revert, deleting the partition would completely remove windows on that drive. (I.E. needing to completely reinstall Windows, which could be very annoying)
There is a manual partitioning option in the Ubuntu installer (called 'something else' when you select the drives).
This article should be of some use: