This is my personal opinion:
First. It was stated, quite well, that there would be a single partner choice and no availability to change partners
Second. If partner changes were indeed available people could abuse that system to obtain specific items or speed through certain levels due to level differences between new partners.
Third. Integrating a partner change system would, in all honesty, screw the coding and bring in a lot of bugs and glitches that are not necessary to the game.
In conclusion:
If you are not happy with your current partner either A. Continue in as a solo ironman and prove you can do it
Or
Remake your EIM and find a partner who will be helpful and active.