You can do a pass exchange if your pass conditions allow it (see your confirmation mail):

https://www.interrail.eu/en/interrail-passes/ordering-info/refund-exchange-policy

Your reservations are entirely separate from your pass. Whether you can exchange them and how, depends on which TGVs (which route) and where you booked them.

IF however you have already used some traveldays, then an exchange is not possible anymore.

In some=not all countries one can-sometimes-upgrade from 2nd to 1st for THAT sector by paying the difference between normal fares-the cost of that will most often be astronomically high.

In France, there's a separate reservation price for 1st class with a 2nd class pass. The price of that is quite acceptable; I've seen €19, €30, €35, €36, €46 (seems to be distance-based).

I go to Switzerland for 5 days. Now i have a interrail plus mobile ticket (not activated).

I have reservations for:

Thalys Bruxelles-Paris 

TGV Paris - Zurich

TGV Basel - Paris

Thalys Paris - Rotterdam 

I will spend 3 days in Switzerland and 2 days for travel from the Netherlands to Switzerland and back. I want to change my 2nd class interrail to 1st class but i want to keep my original reservations.

What do you recommend me? Keep my 2nd pass or change it so I can travel in Switzerland in 1st class? And go to there in 2nd class TGV.

Keep your 2nd class pass I would say. Why do you want to travel 1st class in Switzerland?

I go there from the 2nd of January till the 6st of January, during holidays so maybe trains are full. I don't know never been to Switzerland before.

But in this situation I think I will stay in 2nd class and keep my pass otherwise I have to struggle with the reservations which I already made. 

What do you think?

That's not likely. There are enough trains and they run frequently. I've been to Switzerland many times during that period and finding a seat was never a problem.

Oke thank you!!

In any case, if you change the pass the reservations will remain where they are, and valid in combination with your new pass. They have their own refund & exchange conditions. It would be a waste to travel in 2nd class with a 1st class pass, but that's allowed.

Changing reservations should be possible, albeit sometimes at an extra cost. Seemingly €20 on Lyria, Thalys is 75% refundable but allows for 1 free change, which might enable you to change the class - not sure, so check before you do anything. In any case will be depending on availability, and for the dates you mention they might have sold out.

----

While I generally buy 1st class passes for the extra comfort, I agree with the above that travelling in Switzerland in 2nd class should be perfectly fine, so I wouldn't change it for that. Lyria in 1st class is also not worth the higher price in my opinion, and the Brussels-Paris Thalys is a short ride. If you'd take the Basel-Utrecht ICE I would have recommended it.

In any case, enjoy Switzerland! 

Oke thank you! I will keep my pass and take the 2nd class. I prefer travelling over Paris instead the ICE Utrecht - Basel because of the long travel time. And in my opinion TGV are more comfortable than the ICE.

But the reservation on the ICE would be much cheaper 4.50€ in second class.