by first week of flower what exactly do you mean?
first week since you changed the light schedule? or first week since there was noticable bud and pistol formation?
if its the first week since flip they should be stretching, keep giving veg food and maybe verry light bloom food mixed in....
if its first week since noticable bud formation then they are going to stop growing vertically and start working on bud production, this is when you want to slow down on the veg food and pick up with the bloom food, dont cut out the veg food completly yet though as they are still using a small amount of Nitrogen..
either way as long as they are nice and green and you have adequate conditions you should be fine. these things take time man just gotta have some patience.
as far as "upping the food" goes there is never a "given" time to increase your feeding, if the plant is staying nice and green then keep the feedings the same, if you start seeing some deficiencies then its time to check your PH to make sure thats correct, if it is then increase...