The font issue is small potatoes. What is driving the replacement of signs is the change in federal retroreflectivity requirements.
Starting in January 2012, all jurisdictions have to have a plan in place to make sure all of their regulatory signs (stops, speed limits, yields, etc.) meet the requirements. By January 2015, the plan has to be in place for other types of signs (street flags, parking). Some signs are exempt from the requirements.
I am working with several agencies to make sure they meet the minimum requirements. One is a large city with roughly a quarter million signs in their inventory. The smallest is a small city with 10,000 signs.
There are several ways communities can meet the requirements--a) direct measure of the signs with a reflectometer (a pain); b) setting out a reference sign for each sign type, when the reference sign wears out, all the examples have to be replaced; c) having a trained spotter over 60 years old go out in an SUV or pickup and observe signs. There are a number of other ways that communities can meet the requirements.
In my experience, those communities that have been planning for the new rules will do okay. Sign life is ~10 to 15 years. But the standard deviation is huge. South-facing signs wear out faster than others; stop signs in some locations are lucky to last two years. Those that have not planned ahead are in for a surprise.
Why are there new rules? The driving population is aging. The current sign regulations are basically set for a driver that is 20 to 25 years old with good visual acuity. The new regs will (hopefully) make it easier for all drivers to see and act on the signs within the right-of-way.