With fluorescent lights, especially ones that are specifically made for growing plants, I don’t have any problem using 6500K the entire grow, after all the color of sunlight is 5800K on a clear day, and 6500K on a slightly cloudy or overcast day, and this is true even in the fall/winter when cannabis flowers outdoors naturally.
It is true plants can use more red, especially during flower, and this is why people use a light more in the 2700Kish color during flower. And there is evidence that more red light during flower may also help the plant finish earlier.
The main reason with HID growing that many people use the 2500K during flower is because HPS has this color normally, not so much because this color is actually necessary for flower. It is just that too much red during veg can cause the plants to stretch too much for very lanky tall plants with long spaces between internodes. And so it is more important the other way around, not too much red light during veg, and not that the plant specifically needs more red or less blue during flower. And MH tends to have a naturally blue-er light also, and is more often used for veg as it doesn’t have as much lumens or PAR per watt as HPS.
If buying the extra bulbs isn’t in your budget right now, use what you have. Then later you can try the redder lights and share your experienced differences in bud quality and yield results, I’m sure many people thinking of using fluorescent lights their whole grow will like to know your experience.