Software Isolation

Do a carbon copy of the current disk on an external. The use the external or whatever to boot the laptop, erase the HD, install OSX, then use migration assistant to pull in all docs and data from the external copy.

 

I can confirm what stisdal said. I had a PPC (preIntel) iMac with Leopard OS X 10.5, I also had a bootable clone and Time Machine backups on an external hard drive, so I was able to use that hdd with Migration Assistant to transfer everything to a new 2011 iMac easily. It transferred all the Adobe Creative Suite & MS Word serial numbers & I was able to begin using them immediately without fuss. If your grandfather was on Leopard and did have Time Machine backups, you don't need to make a clone too. --But you should definitely consider making a bootable clone of the new iMac after it's all set up.

Also, once everything's functioning, you can click on the program's name in the menu Bar and look under About -program name- or Version Information to see serial numbers.

 

Here's one more recommendation for making a bootable clone of your grandfather's internal drive, using either CarbonCopyCloner or SuperDuper.

It can be either to a spare drive (not partitioned), or even to a partition on another drive.

You should do this in any case, whether you upgrade his software or not, just to have a backup for him if the internal drive fails.

CCC (or SD) is -very easy- to use. There is no better way to do it.