At the moment, the plan is merely a hypothetical. The owners can't make any changes to the CBA without the approval of the players and the NBPA is still searching for an executive director in the aftermath of the Billy Hunter scandals. Once they do, they could agree to open up negotiations right away or they could wait until 2017, when either side can opt out of the CBA.
The players have never been in favor of raising the age limit, but it's also not something their membership has considered sacrosanct in negotiations either. In short, it's a trading chip for the players - they might be willing to alter the system, but they won't just give it away for free. They will want something from the owners in return for moving the age limit to 20-21.
Here's the question - will the owners be willing to take less money in the new CBA in order to create this new system? If you look at their history over the last few lockouts, it's mostly been the story of them taking as much money as they possibly can from the players. Is a higher age limit worth a few points of BRI or loosening some of the luxury tax penalties?
That's what the players will want for such a dramatic change, at a minimum. If the owners are actually ready to put their money where their mouth is, the current system won't last for long. If they want to treat the NBA as a long-term investment instead of a short-term cash-out, there are any number of changes they can make to the youth basketball infrastructure in this country.
Alternatively, they could be using this proposal as leverage in CBA negotiations, something else to shove down the player's throats as they take as much of their money as they possibly can. If the owners really want to cut off the NCAA at the knees, they can do it any time. However, that would mean taking less money for themselves and that's something they've never shown much interest in doing.