Can we still use our old “Men Working” signs until they wear out before we need to switch to the newer "Road Work Ahead" signs? 
The text based Men Working Sign has been replaced with the Road Work Ahead (W20-1) or Worker Symbol Sign (W21-1).
While signs in stock can continue to be used for 10 years after a new MUTCD is issued, the text version of the Men Working sign has not been the standard in NYS for over 20 years and as such, the newer signs should be used. This is covered in part 6 of the MUTCD and
also in both the Cornell Local Roads
Program Work Zone Safety
Pocket Guide and Traffic Sign Handbook. 
If you do not have the newer signs on hand, it would be better to have the men working signs used until you can get new ones ordered assuming the signs you are using have the proper color and retroreflectivity. Do not have a work zone without some type of advanced warning signs.
It is possible that using the older signs could expose the town to increased
legal liability.
Resources
Work Zone Pocket Guide
https://cornell.app.box.com/v/clrp-pb-wzpg
Traffic Sign Handbook – Chapter 8 Work Zones https://cornell.app.box.com/v/clrp-pb-tsh
The relevant sections of the MUTCD is 6F
https://mutcd.fhwa.dot.gov/htm/2009r1r2/part6/part6f.htm#figure6F04
According to the NYS Supplement to the MUTCD, the symbol version is the only one allowed. Do no use the text version.
http://www.clrp.cornell.edu/q-a/214-flagger.html
