The two + a half lines of Pugh/Peughs I am following closely in nearby Montgomery County Maryland (with two also being in Fairfax & Loudon Co VA), all seemed to begin switching to Peugh sometime after 1840. Of course, its a little hard to tell exactly when with many different sources written/transcribed by others, and in many cases members of the same family seem to have been reluctant to switch for whatever reasons. I'll need to go back through some of the land records (especially) to find first origins...I'll try to update:
The short version: My main interests are the the Line of John W Pugh (b 10 Sep 1769Amity Berks, PA, d 8 May 1854), who moves to Goshen, Montgomery Co MD sometime before 1811. One son, Michael L. Peugh seems to be an early adopter. By 1850, he almost always listed as PEUGH (as Postmaster APPT in Goshen MD, several land records, 1850 MD Census, and his Will in 1854).
His widow sues his father's estate after John Pugh's death in 1855/6 and in all of the subsequent legal documents, all family members are listed as PEUGH
Another early adopter amongst the Maryland Peugh/Pughs was Samuel A Peugh (b 1821 Maryland, d 1894 WDC). I believe he is related to the above line...He was a lawyer, claim agent and city councilman in Washington DC. He was part of several large cases during this period, including one Supreme Court case that is still routinely used as precedence in mortgage related legal cases.
His name on marriage records, passport apps, and most other documents has changed to Peugh by 1849.
The last line in MD that I am tracing that seemed to adopt PEUGH is that of Uriah Peugh (b1826 Maryland, d aft 1850). A resident of Clarksburg, Montgomery CO, MD he was Pugh in early indenture/land records, but PEUGH by 1849 in marriage records and other records. All his children completely adopt Peugh.
Hope that helps anybody. I'd be glad to share any more details that would help anybody also interested in these lines, or who might have additional information on them. Best of luck!