Welcome to Powdery Mildew. It is the only fungus I know, that can be treated after infection.
Get a spray bottle and mix 9:1 Water and milk. Spray on your leaves and stems once a day. This worked for me quite well
Cut off yellowed leaves. They aren't working for the plant anyway. This will also promote some air flow.
You most certainly should transplant those plants into 5 gallon pots. Use potting soil, sterilized plain potting soil. Nothing else. No added fertilizer or water holding gimmicks. No compost. Lay the plant and pot on newspaper sideways. Cut off roots coming out of the drain holes, slide pot off root ball.
You might have to give the pot a couple of good wacks if the pot doesn't slide off easily. Fill the new pot with 1/3 of fresh potting soil, firm the soil, water the soil and put your plants root ball in the pot and while you've got the root ball exposed go ahead and scratch up the roots on the outside of the root ball. You could take a knife and slice the outside roots a 1/4 inch a few slices, one on each side? This stops the circling of roots and promotes new root growth.
You'll need to fuss with the stake a bit. I'd leave it in place and plant with plant. Fill up the rest of the pot, firming gently around the root ball. Leave 1", no more no less, of room between the surface of soil and rim of pot for proper watering.
Move the grouping of your plants around; put more space between pots and leaves for air flow. Try turning pots upside down to serve as a stage. This really helps not only aesthetics but great air flow.
Fertilize with a balanced fertilizer of N-P-K. Make sure that the N is the lowest number or equal to the percentages of P and K. 5-5-5 or 4-5-5...the percentages are important if you want reproductive growth; tomatoes.
Powdery mildew is normal for this time of season; late summer and into fall. It will not hurt your tomatoes unless it stops photosynthesis of healthy leaves. Whole milk, skim milk...NOT soy milk. You do not want to be using a fungicide any stronger at this point in tomato production. During the day when you water you should also hose the plant down as well. Powdery mildew doesn't like free water. You'll find this on lots of other plants that are under an eave or roof. If you aren't eating the plant Serenade is a great fungicide to use. Read the directions closely to include the little booklet that comes with pesticides (fungicide is included as is herbicide, miticide and other 'cides' under the label pesticide)
If you've no wind at all I would think about a fan blowing on your tomatoes. Water in the mornings or during the day so that your plants are dry by evening. After you transplant, water then feel the weight of your plant, pot and soil. Pick it up to get a sense of the weight with plenty of water. Check the weight the next day. If it doesn't feel lighter don't water. At first the roots won't be enough to suck up the bulk of the water. Don't water until that pot is obviously lighter. Stick your fingers into the soil and check out the moisture that way as well. When the pot is obviously lighter, water thoroughly. Feel the heft again. You'll soon be watering once a day most likely.
You shouldn't need any more than one application of fertilizer, this late into the season. Use half of the recommended amount. Remove tomatoes as soon as they start to color. Allow them to finish ripening in an airy, dark spot in your kitchen. Not in the frig. Removing tomatoes encourages the plant to make more tomatoes. Best way to preserve excess tomatoes is dehydration. Sun dried tomatoes!