Breach of Contract

I sold the assets to my business one year ago this month. The buyers put 50% down and are/were making monthly payments over 3 years to pay off the remaining 50%. The buyers owed me a large balloon payment on 3/20/14, but didn't pay. The buyers asked me to stretch the balloon payment out over 8 months, which I agreed to. My contract lawyer drafted the amended promissory note (I'm in Calif) and we presented to the buyers. Now they won't sign it and I believe they are in default as I heard they laid off all but 2 employees (from 15 full time when I owned it). My lawyer is out of town until Wed and I'm taking my first annual vacation Wed, so I'm worried a "notice of default" won't be issued in time. If the buyers find a clause to try to get out of paying and file a lawsuit against me, so they don't have to pay, does that put them in a more favorable position than I? I'm wondering if I shouldn't file something in the court through the eFile system this weekend so I'm the first to file.