Changing the casters to a different size also changes the angle of the seat.
Rather than trying to cobble things together that may change something you don't intend to change, I would use something other than new casters.
She won't really be able to see the front casters anyway. Look in hardware store (Lowes, Home Depot, etc) for things like small stick up lights, booklights or lights you put on your head for walking in the dark. Some of those have straps that could be used to attach them to wheelchair tubing.
It would not light up, but you could get reflective striping or sheets of reflective material that can be cut in any shape you want. You could then add that to the wheelchair where ever you want, in whatever shape you want.
Also, the fleece idea someone mentioned made me think of an easy way to cover the wheelchair back that might work. There are no sew blankets that you make by cutting sort of a fringe on 2 pieces of fleece. For a blanket, you make the fringe around all sides, then lay one piece of fleece over the other. When you tie the fringe from the top fleece to the bottom fleece, you end up with a nice, double thickness blanket. I don't know if this would work, but you try doing something similar with 2 pieces of fleece cut to fit over the wheelchair back. Just fringe 3 sides, so you have an open side. If there are things like straps or protruding supports that stick out, just cut enough fringe off to make an opening for them. The cover could even be tied onto the chair, rather than made and then put on. The only problem I can think of is whether the knots from tying the fringe together would be too bulky and rub on anything. Also, check the knots occassionally; I know the ones on DD's blanket get loose sometimes and need to be retied.