If you are a beginner get a few good chess books and go over the games with Fritz 8. I use Fritz 10...
Upgrade from Fritz 8 to Fritz 13 ?

Yes you definitely would. Fritz 13 has been designed for both the club and beginning level player. The new ribbon that fritz 13 helps you find stuff much more simply than the earlier fritz versions. You can't play games against a book and thats where you're chess will improve the most by playing games. Set fritz 13 to a very bad player on up to a grandmaster and have fritz analyze what you did wrong.
Toss fritz 8
I still have Fritz 8. Didn't know that there was any upgrade plan. Surely you have to buy Fritz 13 at full price???

Ask yourself what do you think is missing from your 2 existing programs?
As a fellow beginner, my best "investment" was (and is) spending time on tactics. But that's not as nice as buying shiny new things
The newer ChessMaster versions come with quite a lot of tutorials as well as more "human" player personalities than Fritz.

I run a chess club and teach chess to students of many levels. I agree that Fritz 13 is majorly irritating. Fritz 12 is on sale for $10 on Amazon. That is the last good interface.
Horses for courses. I find the new ribbon UI of Fritz extremely disfunctional and irritating.
I would surely stay on Fritz 8 if the latter could accept pgn files as input (it doesn't) and have a less crappy engines module.
I have decided to improve my chess - I am barely above beginner - we have in the house chess master 9000 and Fritz 8 - am I going to get any benefit out of Fritz 13 that I don't already have with Fritz 8 ? Many thanks for any advice.