My Fluval Plant 2.0 lights broke on the 14th June 2019.
They were under warranty and replaced with Fluval Plant 3.0 free of charge on 28th June.
What I learned from a two week blackout.
1). This kills all algae.
2). This will kill most plants.
3.) Due to algae and plant decomposition you get a massive build up of organic waste in the tank.
4). When you reinstate the lights you get a massive algae bloom - hair, fuzz, BBA, BGA, diatoms - you name it.
I know no one is suggesting a two week blackout, but here is what I would do if I planned even a 4 day blackout.
A). Remove / Rehome as many plants as possible.
B). Do big water changes every day, or at least every other day.
C). Reintroduce as many plants as possible on lights on.
D). Do water changes every other day for the first week from lights on.
In truth I think i would prefer to break down the tank, give it a 100% clean, including hardscape and substrate - and start again.
You can achieve this in a few hours and probably end up in a better place.