Ok then.
You will need new
New bulbs
New bulb sockets/holders (the ones you have may be adjustable, many are)
You will know they are adjustable because there are two mounting screw holes in the base of the holders. The front set is for 250's and the back is for 150's)
New ballast. If you can get two single 250 ballast. A dual ballast is nice but single is nicer because when one goes out you don’t lose all your lights. Cost is only slightly higher and allows for a more flexible system.
You MIGHT need new wire. (it is important to ensure the current wire can handle the extra power.)
Other then that you should not need anything. MANY MANY times the only difference in the 150 and the 250 all in one units like you have is the ballast and bulbs. Sometimes they have an addition of bigger or more fans but for the must part everything else is the same. You can call the MFG and ask the tech dept what if any differences there are in the 150 vs. the 250 other then wattage. if none it will be a straight swap. If there are then you will need to decide if they are important enough to worry about.
When you get into it and find you get stuck on something feel free to give me a shout and I will help you out. I have done a ton of DIY lighting, retros and swaps like you are doing