@xivarmy, how do you change the message so that it isn't confusing for the vast majority of players who have no concept of MMR? The average player is going to be quite confused by a message that says "You've advanced" when it is clear to them that nothing has actually changed from their perspective. The only option I can see is to unveil some of the hidden aspects of the system - such as publishing the MMR, or giving identification to the different battlefields within each league ("You've moved up from Silver-4 to Silver-2").
"Great week! Keep it up and you'll advance soon"
"Well fought! We're making good progress"
"Nice battling! We're holding our own"
"Tough competition! Maybe we should look for some reinforcements"
"We're getting destroyed out there! We need to remobilize immediately!"