Again, on Changing the Program.
Why do you want to change the program?
This question is critical. If you want to change a program, you need to know why - why you chose the program, and why you want to make the change.
First, ask yourself this question:
Is the program the right program for you?
Ask yourself if the goals and approach of the program match your goals and your ability to approach it. In other words, can I do this and will it get me where I want to go?
Don't select a squat-based program if you cannot squat, or choose an endurance program involving running if you can't run, or choose a 4-days-a-week program if you can't train more than 3 times a week.
Don't select a program just because you like the name, like the trainer who wrote it, or like the exercises in it. Select it because it meets those two criteria - you can do the program, and it will get you to your goal.
Choosing a program that accomplishes A and B when you want result C is like buying limes and trying to make lemonade out of them. It doesn't matter how good the limes look or how inexpensive they are, they won't get you the lemonade you wanted.
Once you have the program you need, you may feel like it needs some tweaks or changes to get you exactly what you want. So ask yourself:
What does changing it accomplish?
What do you intend to accomplish by making this change? Is it changing for change's sake, changing to add in extra exercises you think you need, or to avoid doing something difficult?
If you are adding in extra, take a hard look at what is there already - is the program really lacking that aspect of training? If so, is there a reason for it?
For example, Starting Strength lacks conditioning work. This is on purpose, because its creator has you squatting three times a week and feels that conditioning work will interfere with your gains.
Or perhaps the program just appears to lack that aspect. Many of my programs contain unilateral training, many standing exercises, and exercises while kneeling tall. They may, at first glance, lack direct ab exercises. But your abs are working on all three of those areas, which comprise almost 90% of the workout. So you might look at the program and say, it needs abs, but in fact, it's mostly abs already.
And so on.
If you make a change, it has to be to address a specific need the program doesn't address, and which won't undermine the intent of the program. If it violates either of those, you are better off finding a different program.
The trick is not finding the "best" program, but finding the program that is the best for you and your goals right now. If you like a program, and you'd like to try it, save it for when it is in line with your goals. Exercise is a lifelong activity; you have plenty of time to do that program when it aligns with your goals.