Yes, – but first make a copy of the library just in case! (you can do this by copying the endnote .enl file and the whole of the .DATA folder to a new location – don’t put the enl file IN the .DATA folder though. keep them in the same place).
But you should be able to use the libraries you created in X3 with Endnote X9 and X9 is compatible with Office 2019.