Thank you for contacting support. Each Aspose license has an expiry date and it is valid for all API versions released from the date it is issued until the date it expires. By default, it also works for old releases, but we do not guarantee that it will work with all of the old Aspose APIs.
The old license file works perfectly with older releases forever until or unless you don’t upgrade Aspose .NET library version. If you upgrade Aspose .NET libraries and use an old license file (it will be considered as expired license if its expiry date is less than the upgraded Aspose library release date), then you will face red evaluation watermark along with many other limitations. Please also refer to this help topic: Evaluation Version Limitations (Aspose.Pdf for .NET )