What if the Soviet leadership thinks it through; decides that Afghanistan is a ticket to nowhere good, merely sends some advisers and air support to Kabul.
At the same time, they see a chance to gain a strategic foothold in Southern Africa, with access to warm water ports and a corner on several strategic minerals. Furthermore, the propaganda value of taking on apartheid South Africa is priceless for them.
By big in Angola, we're still talking about a much smaller commitment than Afghanistan; call it a VDV airborne division supplemented by an East German motor rifle brigade and a battalion of naval infantry, plus some specialized support units and special forces detachments. Combine that with what the Cubans are sending already, and UNITA and the SADF are going to have their hands full very quickly.
My guess is that this triggers much larger support from the US than in OTL, routed through Israel and Taiwan-perhaps sending the IAFs older French Mirages to South Africa in exchange for more modern F-16s. They'll also likely want (and get) modern antitank and antiaircraft weapons. At what point does the US break down and start shipping stuff directly traceable (think Stingers).
Thoughts