replacing old t5 bulbs

So here is my question. I have 4x54 t5 just got my new bulbs in mail today(I went with all ati bulbs). I replaced one of the old blues with blue+ and one of the whites with a coral+. How long do I need to wait to replace the other two bulbs, and was it ok to replace two at a time because it really looks looks nice.

For me, when it's time to swap bulbs, I change out all of them at one time, then shorten my photoperiod to 1/2. Each week, I then add a 1/2 hour to the photoperiod until I get to my original setting