So if you just changed units 3rd week into the semester, do all your classes just change for the one you replaced. Grades won't be affected?
Not sure what you mean by grades exactly but it wont count as you having done that previous unit. If you miss any assessments though because you transferred 3 weeks late, that might be a repercussion on the new unit. It depends. They might be willing to let you do them late or something. Half the reason there is a limit to change into new units is due to that, there's a point where you miss so many assessments and lectures it'd be very hard to do well.
Ideally, you should really work these things out by the end of the 1st or 2nd week.
You can drop a unit much later into semester but you cannot pick new ones up at this stage. I know people who have done this when they realised that the subject isn't really their thing at all and they dropped out so they wouldn't fail it and it wouldnt be counted on their record (plus to avoid all the stress and work too).