My personal thinking about changing exercises is that you need to stick with movements for a long time BUT you need to change them often!!! Let me explain.
The way I see it. The main lift of each session should remain the same for a long time (8-12+ weeks) because you need something to evaluate your progression objectively as well as to assess your weaknesses.
The way you should work is:
2. Plan your progression on your main lift for the mid/long term... doesn't have to be a full periodized scheme with percentages but you need a plan on how you will train that lift and what you want to accomplish on it.
2.Do the main lift as planned
3.Use the performance on the main lift to evaluate what are your weaknesses (part of the range of motion, a specific muscle group, etc). Not just how much weight you are lifting but where is the sticking point and technical issues.
4.From that evaluation you select your assistance exercises for the day. The goal being to address the problems you just diagnosed from doing the main lift.
So the assistance work can indeed change at every session if needed. It has to be something that fix what you feel is holding you back in the main lift.
The main lift of the day helps you select the exercises that will stimulate strength and size growth the most by addressing something that is lagging.
Then you use the progression on the main lift to evaluate if your exercise selection is adequate... if you are not progressing then you might not be selecting the proper assistance work.