Both .sai and .sai2 files are images/drawings made using Paint Tool SAI, albeit from different versions of the software. The only way how to create .sai2 files from original .sai file is to use the latest versions of the tool and simply resave your files.
There is really no other way how to do this as no 3rd party tools support SAI formats and we also do not have information about any kind of utility capable of sai to sai2 conversion.
Updated: August 29, 2023