I’m creating a add on app, I would like to be able to change (stock) planes
and altitude at least once in the game (challenge) … If I can do it within
one challenge it would be much more emersive.
You can change the aircraft altitude just fine, using the SimConnect API. Have
a look at the published „simulation variables“: whenever it says „writeable“
you can change it 
You can spawn “AI aircraft“ (controlled by you, the AI or
„static“) with any available „container“ (aircraft/livery), and change that at
any time (remove old aircraft, add new one with new livery/model). However I
am not aware of any SimConnect API that would let you change the USER
aircraft. Given the fact that you can do so in development mode in MSFS itself
(via the given menu entry) there is definitively functionality in MSFS that
would let you do so while in flight - it just doesn‘t seem to be exposed via
SimConnect (unless I am wrong).