There's a very short window that opens up to allow a light change but you have to spot it quickly and react fast because if you miss it you won't reap the rewards for your entire crop. When I say short I mean short - It is less than 1% of your total life cycle! There's a lot of theoretical physics that has to do with it all but I won't bore you with it, instead I'll tell you signs of when to do it.
The first sign the window has opened is when the plant has grown up to 4 nodes. Second sign is that the plant is still alive, always a good idea to double check. The next sign is to check how many fingers your fan leaves have. Do they have 5 or more? Great now react fast, you only have about 7-10 weeks before you've missed your opportunity and the window closes. See how short it is, it's less than 1% of the average humans life cycle so keep a keen eye so you don't miss it.
TLDR; as long as they aren't seedlings you can change the light any time. Changing the light late in flowering however will not show much improvement in your yield and would likely just cost more to run, but no harm done at all and your next yield will show improvement.
