All the demos I've seen, the part starts at room temperature, there is an array of flame nozzles with controlled motion that move from one end to the other, and there's almost always a flood water (might be a synthetic quenching fluid, but I doubt that's common) quench that follows the flame at a fairly short distance. Travel speed is adjusted to bring the ways to the desired temperature at the desired depth.
Hard case depth varies a lot with the parameters (e.g., travel speed) the manufacturer chose, but 0.025" to 0.040" before finish grinding would be a reasonable range. That means a moderate regrind job may preserve some of the hardened case.
Whether rehardening is "required" would depend an awful lot on the specifics of the machine involved and how many years additional use, and the severity of expected use. I'd generally say "not required", because if you plan to put another 3 to 5 decades of hard labor on the machine, you're probably better off buying a new and/or additional machine. If the machine is being reconditioned for a home or recreational shop, the answer is "absolutely not required".
There is probably someone who can make a stab at flame-hardening for you, but it's not a common service, and unlike the original manufacturer they certainly have not dialed in their procedures on your specific model of machine. So, some risk involved. If the original manufacturer is still in business, and if they recondition their older machines, probably best to send it to them.
