You're right, you definitely need to change your program if you've been using it more than 4 - 6 weeks or so. Otherwise your body will adapt to the exercises and you won't continue to progress.
Since you like the basic format of your program, why not keep the overall structure and just swap out exercises? Pick new ones that work the same muscle groups and plug them in to replace the old ones. For example, if you're doing step-ups, substitute lunges. If you're doing triceps extensions, substitute kickbacks. For front raises, substitute lateral raises and so forth. Keep changing and swapping to keep your program fresh.