The problem with many (counter) arguments here is that they are based on older contracts or agreements, but overlook the fact that these can be amended by new Letter Agreements between Treasury and FHFA (if both agree) to reflect the circumstances and needs of the time. So the whole thing is in flux. There is no point in being legally stuck in the 1980s.
Also, JPS cannot be converted to commons by contract. You can find it explicitly printed there. However, they can - if 2/3 of JPS holders agree to such a conversion. So anyone who searches the old contracts and finds some "proofs" there is not revealing eternal truths.
Nevertheless, the JPS holders remain contractually protected because changes to the contracts can only be enforced with a 2/3 majority. If the majority finds these changes disadvantageous, nothing will be changed.