I guess there are two issues here: cost (say your INF is 90% ready and you add ART) and gearing bonus
While for single unit addition (and corresponding change of cost) IMO should not be big issue, imagine that it is your 5th infantry and now you want to start adding new brigade, or maybe you dumped Engs in favor of ART. What gearing bonus you want to apply? Imagine that after 2 more INFs ready you want to go back to attaching Engs do your troops. I guess that you would love to keep old Eng gearing bonus?
Moving aside issues what does it mean (in practice), it would mean that each unit in production queue would need to keep possible gearing bonuses for all brigades (even those not researched yet).
Edit: and even worse thing is about build time: say your INF will be ready in 5 days and you add ART. Say that even you are ready to play enormous cost of building ART in 5 days - but how realistic is it? So alternative: your INF is delayed by 100 days waiting for ART (should they not loose their gearing?)