1. depends on the lights. If the nav lights go out, don't need them during day ops anyway, continue flight as normal.
2. VFR you don't often file a flight plan unless crossing an international border. If you have, and you want to change plan you can cancel and resubmit, or change as you go. Really depends on the circumstances.
A PPL is valid for 5 years, this is a paperwork thing only and I don't think it will change with EASA.
You have a rating to fly on that licence. This will be a Single Engine Piston (Land) rating - SEP(Land). This is valid for two years. You can keep this going by flying experience alone with 12 hours in the last 12 months of the rating, one of which must be with an instructor. If the instructor is happy, an examiner will sign your licence for another two years. If you do not meet the experience requirements, it's a proficiency check with an examiner which is a bit different from a full flight test.
You could then add a multi engine rating, IMC rating, any rating really. They will all have different periods of validity, and different requirements to keep them current. The licence itself will still need to be renewed every 5 years.