Lets say Laptop A has a 320Gb Hard drive And Laptop B has a 160Gb Hard drive
I want to move Laptop A's hard drive (with the files in Laptop A's Hard drive) into Laptop B and Laptop B's hard drive into Laptop A
Possible complications of plain switching:
That windows reactivate thingy
Driver problems

Johnny's solution:
Send files into external hard drive
Format both drives
Switch Drives
Reinstall Os and transfer files from external hard drive

Who has a better idea because i want to do the plain switch and try to fix it then actually...move files and format...because its too much work...

This might work just fine, but it depends on drivers, and hardware, if the laptops are made by the same people, it should work a lot better.

Back them up first, then swap them out without formatting to see what happens.

Your Plan was perfect. Or, If you don't have an external HDD, use a Backup Service to back up the files. Apart from that... It's a perfect plan.

Send files into external hard drive
Try each drive in the corresponding laptop (if it works then )
Format both drives
Switch Drives
Reinstall Os and transfer files from external hard drive