ExtendSim 2024 is upwardly compatible and will read in models built in legacy ExtendSim versions 10, 9, 8, and 7.
If you want to open a model that requires a custom library created in a legacy version, before opening the model, open the custom library. From within ExtendSim, go to Library > Library Tools > Compile Libraries... Select your custom library and click Compile Selected Libraries. This will recompile the library creating an ExtendSim 2024 library with a .lbr extension. Then, open the model.
Note: ExtendSim is not backwardly compatible. Models and blocks you build in ExtendSim 2024 cannot be read by earlier versions, such as ExtendSim 10 or ExtendSim 9.