Water changes aren't going to hurt floating plants. Their roots aren't super-delicate.
There's nothing special you have to do when swapping water. I drain out about 15%, add the same amount of remineralized water back.
Don't over-think it or make tank goodies more complicated than they need to be.
For curiosity's sake, how do you manage the floaters when you're doing water changes? I've only ever had duckweed in my 8g but I'm upgrading to a 20g and wanted to try frogbit or RRF since I've got more room now, but I'm worried that I'll just end up hurting them with weekly water changes and that the roots will suffer because of it.