Can you rearrange workouts because of a hard day?
Short answer, yes you can!!
Now for the long answer. It depends on your goals ultimately. If you're weeks out from a competition you don't want to rearrange workouts, but if you are just training to be healthier it's not a problem.
Life gets hard and it gets busy!
To make sure we get the best workouts and best results from workouts you can do 1 of 2 things.
First and the easiest is to consider the hard day of work you just had to now be a rest day and replace your rest day later that week with the exercise you were supposed to do.
Second, if you still have some energy and can manage to get a workout in then going to a workout that is a little easier such as arms instead of legs would be something to do. The reason you may do a different workout is because you aren't that tired, but you worked outside all day using your back and legs and you need to workout something else since they're already fatigued.
Of course the ideal thing to do is stick to the workout schedule your trainer has given you, but most trainers should understand that life happens and that things will need arranging from time to time!