There are change machines for the BART if you're lucky enough to be at a station that has both BART and MUNI service, but all this will do is take your $10 or $20 bill and spit out $5 bills - which is good enough for the BART ticket machine but useless for getting on the MUNI. The BART ticket machine actually has a function that will take a $1 bill and give you four quarters (which is nice of them, given that it's irrelevant to the BART service itself), but again, you have to have a $1 bill for this to be of any use.
We actually asked a guy at an information booth and he readily acknowledged that if you want to ride the MUNI and you don't have anything smaller than a $5 bill you are out out luck.
We ended up leaving the station, buying something nearby and going back to the station, where we then used the BART fare machine to break the $1 bills into coins (remember: the fare attendant can't take any payment; you need something to stick in the turnstile or you don't ride). It was not the first time I felt some appreciation for the TTC, where booth attendants can actually enable you to ride the service they work for.
My message to MUNI: Get your act together.