That is quite natural – and clearly documented in many places.
AxCrypt 2 can open AxCrypt 1-files.
AxCrypt 1 cannot open AxCrypt 2-files.
AxCrypt 2 will auto-upgrade AxCrypt 1-files to AxCrypt 2 format when you open them.
If you wish to revert to version 1.7 (then at least get the most updated version from http://www.axantum.com/ !), download the standalone / portable version of AxCrypt 2 from https://www.axcrypt.net/, and decrypt the updated files and then re-encrypt them with version 1.