There is a Cores option. Setting this to one less than the total cores you have in your CPU. Will typically allow your system to still be largely useful. Where otherwise CHDman would bog down the system to where it's frustrating to use. For those with a single core. Once you start running the Update operation, you can set the priority of CHDman. Setting it to Low will typically allow you to still use your system, while CHDman works away in the background. Of course setting a lower priority & fewer cores will of the obvious side effect of taking longer to complete.
Now if the source & destination folders are set the same. The original CHD will be replaced, if a different destination location is picked the original CHD will be left unharmed.
Info & Verify options are carried out on the source folder; so if you have just carried out an Update which involved a different destination, there is a Swap button to swap source and destination entries.