I want to upgrade to Windows 10 but I don't want to decrypt my system partition because it takes 21 hours..
Is it possible to upgrade without decrypting?
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If the system partition/drive is encrypted and you want to reinstall or upgrade Windows, you need to decrypt it first (select System > Permanently Decrypt System Partition/Drive).
Source VeraCrypt FAQ
I and many others had success using Windows 10 media patcher for upgrading VeraCrypt-encrypted systems, you should try it.