Since the update from 18.12.17 ScummVM is used. To make your old save files compatible, you will have to rename and move them to game's installation folder, default path is:
C:\GOG Games\Riven - The sequel to Myst\
C:\Program Files (x86)\Galaxy\Games\Riven - The sequel to Myst\
Save files should use the following naming scheme: riven-000.rvn, riven-001.rvn, riven-002.rvn, etc.