It depends on what needs modifying. If it’s just a couple of simple changes then it should be OK. With the SMF import script I found it only did a partial job for me so I made significant modifications to it and re-ran partial imports dozens of times and full imports quite a few times too. It’s surprising what you find didn’t work quite as well as you thought when you get to compare the new forum content against the old content! I created a couple of scripts that would wipe the previous test Discourse system, install a new copy of Discourse and then start another import run using the latest version of the script.