Okay, basically, there was a general, David Hunter, who OTL issued an order emancipating salves in 3 states and tried to form the first Colored Regiment, the First South Carolina. It was refused due to political pressure from border states.
Now, my idea had CSA General Cleburne, famous for considering arming slaves late in the war, captured at Shiloh. I had his comments comparing slaves tot he noble warriors of Roman days a la Spartacus get out and then blown into propaganda by the Union, which caused tghe 1st South Carolina to exist, but not fight in the South. However, you could probably use this idea more convincingly.
The idea si that Hunter would be allowed to form his unit, but in a political compromise they wouldn't be allowed to enter a border state. This sounds like something a slightly more radical LIncoln would allow, as he would start to be okay with African American servicemen earlier but would still be worried about a bunch of ex-laves marching through border states.
The point is that Hunter's Colored Regiment is sent up to Minnesota in time for there to be just enough delay that the Dakota of Western Minnesota don't rebel. An annuity payment hadn't arrived in time OTL, fight. OTL, the Federal agent
wouldn't' distribute food without payment, but my idea was that Hunter's troops are there and he chooses to distribute food anyway because he sees the plight as one of discrimination and that - having been denied the right to use his troops against the South - he's got a bit of a rebellious streak himself against orders so he says "let's show them by giving out the food." This distribution of food means that a move to rebel is delayed long enough for the annuity payment to arrive and for the Dakota to then pay for the food that was given out, thus allowing thigns to be patched up. For how long is uncertain, but it could lead to peaceful existence for much longer, perhaps permanently. (The idea of the TL was to make things better in a number of areas.)
I then had the idea that Hunter's group would be sent to Colorado and stave off problems with the Cheyenne, too, preventing the Sand Creek Massacre, before attacking through West Texas. (Since, you know, thre weren't any border states there.
) This would prevent the Cheyenne War, too. And, also fulfill your idea of small changes at first; accepting the regiment early instead of rejecting it but still not allowing them to fight where they could rile anything up in border states, let alone start freeing all the slaves in 1862.