The volume rendering has changed in 2018.1 to improve the sampling and so on. If you need to render a pre-2018.1 scene in 2018.1, you should enable the "Emulate old volume behavior" option that you can find in the kernel nodes. Once enabled, the 2018.1 volume rendering should match the 4.02 one.
Computer Graphics Software Developer
Octane plugin development and support: LightWave and Houdini