I have a CRSP stock dataset that goes up to december 2013. I'm trying to append yahoo finance data to it in order to bring it up to the current day. However, there are ticker symbols that once belonged to a different company than they do today. For example, until 20121015 AAC was the ticker symbol for Australia Acquisition Corp, but now it's the ticker symbol for AAC holdings, a pharmaceuticals company. I don't have a source of permnos for the yahoo finance stocks, so that's not an option for me to keep track of which stock is which. Is there a minimum interval of time between when a stock symbol is no longer used by one company and begins to be used by another?