Question about replacing expansion tank....

I have a 2001 Escape XLT. The engine coolant light has been on for the past year. I had the coolant flushed and replaced which didn't solve the problem. A mechanic said the sensor in the expansion tank is bad and quoted a price of around $350. I've found a new tank for $80 and was going to do the fix myself. The problem is, my Chiltons manual shows that I have to drain the whole coolant system first before removing the tank. Why wouldn't I just need to drain the expansion tank instead of the whole system? Or can I plug the two hoses when I take them off and then just replace any coolant that I lose when I unhook them? Why drain the whole system???

You're right ... no need for a total draining for such a small task. A pan to keep the small bit of fluid wasted away from pets is all that is needed.