Upgrading Gates CDN to CDX-EXP First of all I will start with the fact that I'm completely new to the world of hardware related to bike parts.

But I must say I ride my bike a lot. My everyday commute is roughly 40-50 km. I drive a Pegasus Premio SL belt 2019 and it is a city bike. I bought it almost 6 months ago. As of now, I have rode more than 5km with it. As far as my knowledge goes, it is equipped with a Shimano Nexus 8 speed hub with a Gates CDN S150 belt drive system. I don't know how many teeth I have in my belt (sorry).

Recently I saw that my belt started to chip its teeth away (of course wear and tear). Therefore, I'm planning to switch to a Gates CDX-EXP version to avail myself its robustness (at least that's what the website claims ;)). So, here I am.

I would like to know which combination of sprockets (front and rear) should I use and how many teeth should be in my belt and why. Moreover, I would like to know the meaning of teeth in the sprockets and belt. In addition, I would like to know whether I can use the re-use stuff from my old CDN S150 crankset.

This question is mainly because I tried searching for combinations on google but not with much success (I'm bit lost, because I haven't had much experience with it, yet). I of course read their manual, but it has different combinations and honestly, I'm a little bit lost.

Finally, I would like to have your feedback on whether it is worth upgrading.
regarding the amount of teeth on the sprockets/crankset:
 The ratio between the amount of teeth on the rear sprocket and crankset determines the gear ratio of the bike. Even though you have a gear hub the ratio between the sprockets will still determine your gear ratio. The smaller the sprocket in the back/the bigger in the front the higher your top speed is, but keep in mind it will take more force/less revolutions to keep your bike rolling at the same speed when in the same gear with such a sprocket ratio (small in the back large in the front).  

check out the "gear ratio" section: https://www.whycycle.co.uk/buying-a-bike/bike-jargon-buster/bike-gears-explained/
general info regarding gears: https://www.sheldonbrown.com/gear-theory.html

If you want your bike to maintain it's current gear ratios either re-use the original sprockets or pick new sprockets with the same ratio of teeth (back/front).

the amount of total teeth on the belt should not really matter as long as: A) your belt isn't too short as in you cannot get it around both sprockets even when the wheel is adjusted so that it is the position most to the front of the bike as it can be B) your belt isn't too long as in you cannot get the belt tight enough with the available adjustment range (back to forth adjustment of the rear wheel) without the rear wheel falling out of the dropouts

A higher number of teeth on a sprocket means the force on the belt is distributed over a larger number of teeth. Which will reduce the wear rate of the sprocket and in theory also of the belt.

Regarding compatibility: This page suggests the CDN and CDX-EXP parts are compatible.
https://willem.com/blog/2018-02-24_creating-the-ultimate-commuter-bike/#continue

Judging from the story on the website above I'd say upgrading to CDX-EXP (especially the stainless steel instead of polymer rear sprocket) is worth it. As can be seen on the website the polymer sprocket doesn't seem to hold up all that well, especially when riding in harsh conditions.

This is after just 1800km according to the website:

CDN vs CDX-EXP sprocket: