Reloading a MacBook with applications My daughter has a MacBook, 10.6.8 and we have the install discs. 
It's gotten to that age, 5-6 years, where it runs sluggishly, and I'm thinking to wipe the drive, maybe replace with an SSD, and reinstall the OS.

I am looking to be sure that the applications that have been activated by serial number continue to work. Is there a simple process to do this? In the very old Mac days, the serial number / keys were stored in a preference folder. Now, I'm not so sure what pieces need to be saved to move software from an old drive to new. 

I'd go the CCC (clone) route, but a full image would probably take the sluggishness with it. Trying to clean up the next version of the drive.
Take a single Time Machine backup, just in case.

When you wipe the laptop, use the Time Machine drive to selectively (manually) restore certain items.  Information and files related to application licensing will most often be located in:

~/Library/
~/Library/Application Support
~/Library/Preferences