If you are capable of soldering small electronics, it might not be difficult to swap the driver yourself. Otherwise, you'd have to find someone who can swap the driver for you. Since your son gave you the light, it might have some sentimental value and be worth paying to modify, but the cost might be more than the torch cost to begin with.
For some situations it's preferable for a light to come on low first. For example, in the outdoors, away from ambient light, high mode is usually not necessary, so it's typically best to be able to try low first. Other times, high mode first is better. It might be better/cheaper to buy a second light that comes on high first and then carry the torch most appropriate for the situation.
I think the body used for your torch is used for a wide variety of lights (I have an empty body just like it somewhere, and I've seen others like it), so you might be able to find a similar torch that comes on high first, and then swap out the driver and LED in a single piece without any soldering. That would allow you to carry the torch your son gave you, and have it operate as you desire. Plus, you'd have a second torch!