Wait, just to make clear that I understood that correctly: Instead of receiving officers free by teching up (something that has looked quite hypocritical since the removal of OKW's teching boons, and especially when looking at Ostheer and Soviet teching), officers would have to be upgraded from Riflemen separately of teching up?
On one hand that offers so much interesting possibilities (tying the exact officer upgrade to the veterancy level of the squad that is to be upgraded, i.e. a vet1 Rifleman squad can only become a Lieutenant, while upgrading to a Major requires a vet3 squad), but on the other hand that would require significant rebalancing of those officers, considering that a Major has significantly less combat impact than a Rifleman squad (thus reducing the probability for players to upgrade to him).
There are many way to implement promotion of riflemen to other infantry or officers. The simplest for start would be to have the additional option to promote a riflemen to LT for lets say 75 MP 50 FU once riflemen hits vet 1 while losing lets say 50% of his vet.
A similar principal could apply to ranges, paratroopers, assault engineers, pathfinder and so on...
Or one could even try to have RE being able to be promoted (even to riflemen) but in that case veterancy would probably have to go up to 2 or even 3.
If one wanted to be really creative, one could try to have USF start with a single riflemen, have riflemen locked behind an officers and allow RE to be upgraded to other infantry.
That would give USF a unique feel, create thematic feel of an untrained army, make the need to nerf riflemen less severe since they would hit the field later and allow more diversity builds since one would be able to have more core infantry types by choosing doctrines.