One way to test other instruments is to download demos of other programs which have various instruments included. If you like the instruments in Samplitude Music Studio it can be worthwhile buying the program, when it's at a sale price, as this is one case where instruments work across programs. I did this to find out if the Drum Engine was worthwhile (it wasn't) while I also found World Flutes would be if in a sale.
However as the instruments are just different versions of the Vita sample player some with few samples many are not remotely worth the price. It's better to look for other options through VST. Magix own Independence Plus Suite is a reasonable option as it covers a lot of areas although I waited for a sale to get it half price. Elsewhere there are things like Synthmaster Player which even has a free version with 550 sounds.
In addition to what @Graham-Hawker wrote, are you aware there is a "demo" facility within the Store in MM? If you click on any listed instrument which you do not currently own you will see a white "triangle" in the centre of the image (see below)
If you click that triangle you will hear a demo of the instrument.
As to "exchanging" instruments, it is my understanding that once purchased, no exchange is possible.