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One of the most significant changes that you will encounter with EndNote 20/21, is that libraries created in EndNote X9.3 or later cannot be run in EndNote X9.2 or earlier.
If the libraries you are using have been created in EndNote X9.2 or earlier, when you try to open them in EndNote 20/21 (or EndNote X9.3) you will be prompted to convert your library to a new format. Before you do this,
Once you have made a compressed back up, open your existing library in EndNote 20/21. You will be prompted to convert your library to the new format. When you convert your library, you will see that a new version (.enl file and .Data folder) is created in the same location as your original library. The new library will have -Converted after its name. The original library will retain its original name.
If you are synchronising your library with an EndNote online account, your new library should sync correctly with the online library, although it will indicate it is merging libraries the first time you sync it. once you have synchronised it, check it has been done correctly.