If the process access the variable on start, keep in mind that it might initialize in a different way according to the variable, that is the variable might be used in conditional statement.
For example let's say the variable you are talking about is named
var1 and the process saves it at memory location x0001 and you want to change its value from 1 to 10 without restarting the server.
Let's say you change it to 10 in location x0001. Very good, right?
The problem is, maybe when the process starts, it checks for var1, and it does something like that:
if (var1 < 5) {
var2 = true
} else {
var2 = false
}
In this case you would have put the program in an inconsistent state by changing var1 in memory.
See what I mean?
Did you think about other ways? Do you have the source? If yes, you may not have to mess too much with it, there is probably a initialisation routine that you can maybe just call again, and it will reload the conf without shutting down the service. Might be just a couple of lines of code to make it callable using signals.
BTW, if it's apache you are talking about, you can do apachectl -k reload