I just read a godawful article about a woman with a heroin addiction who at one point started on naltrexone without being clean. I can link you if you like, but it’s harrowing.
But this is just to say, take this extra time to make sure that crap is out of your system before you go on naltrexone. And as @everhopeful keeps reminding you, if you do relapse, be kind to yourself and let your doctor know, so he can adjust appropriately.
Good luck, @anon84763962! I’m sorry that your plans got disrupted, but I trust you to roll with the punches