Do I need to change my sets & reps on Old School Muscle if my progress stalls?

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To ensure you continue to progress, we intentionally change up your exercises after the first 4-weeks on the program to ensure your body doesn't adapt to the training plan.
Essentially, Old School Muscle consists of 2x 4-week training cycles with a de-load/recovery week at the end.
Inside each of those 4-week training cycles, we intentionally increase the sets of each exercise in Week #2 onwards. You'll notice that in all the Week #1 workouts (starting on page 26 of your OSM Workout Guide), there are  3 sets for every exercise
When we hit Week #2 onwards, we increase to 4 sets of each exercise.  This increase in volume will challenge your body to gain muscle to keep up with the exercise demands. 
You won't need to add more sets or change the rep scheme during each 4-week training block, because there are 3 different rep-ranges used every week for the Heavy (lower-rep), Light (higher-rep), Moderate (medium-rep) range workouts.
KEY: you will need to focus on increasing the weights you use each week.
Remember: to make muscle building progress, you need to be  constantly increasing your training weights every time that you can complete the prescribed reps on a given exercise.
See page 11 in the workout PDF - section "Constant Strength Progression = A More Muscular You."
Every time you can hit the proper number of reps on a given exercise, we need to have you increase your training weights for the next workout. Not increasing training weights will be the #1 reason guys stall with their muscle building progress on any training plan.
-Dr. Anthony Balduzzi
Founder, The Fit Father Project
Creator, Old School Muscle