Hi Stephj and welcome to the Project Meditation community,
Yes, you can still benefit from the previous levels, and once you've completed level 1, it's recommended that you keep listening to lifeflow, varying the levels as you feel fit.
If you consider that the tracks are each providing a way of entraining your brain in a certain way, then there is also our everyday life that will have things happening in it that will re-wire the brain differently again, especially if it's something persistent and long term, for example, having trouble with the boss at work etc. So, going back to earlier levels helps to re-enforce that wiring that we were entrained to earlier on... and what you'll find is that, because our brains are familiar with those earlier levels, they'll be more quickly effective than the first time around... if you like you can think of them as a 'top up'
Hugs
Giles