Switching to Bloom - NewB Questions

My plants are 17" tall now. I am going to switch to bloom in a few days. Here are a few questions I have....

They are currently in 2gal pots. Should I transplant to 5gal pots before I switch to bloom? I have also read about flushing the medium to eliminate salt build ups. Should I also flush the soil?... and then give them a clean batch of bloom nutrients?

I also wanted to know when the lights are off does the temp still need to be between 25-30C or is it ok for it to drop down to like 20C?

depending on the space u have to deal with will affect witch pot u use, and don't flush the plants now, u do that 1 or 2 weeks before harvest