Some Weiss relations believe that the surname was previously spelled "Weifs" and then changed, roughly around (apparently) 1865, to the present spelling of "Weiss". Let me be clear - this is hogwash. It's just the way it was written - both the typeface used and the way they handwrote it.
Firstly we'll look at the typeface. They often used a different typeface way back when - in 17th and 18th century English newspapers I've seen "English" written "Engli∫h" and "witness" written "witnefs". Have the words changed? No, just the typeface used to represent them.
Secondly we'll look at the handwriting. Even after the newspapers started using the usual typeface characters that we use today, people were still handwriting it the old way, with an "f" style character. When he was married, in 1828, Charles Nicholas Weiss signed the marriage register like this (I've traced it): So was his surname "Weifs?" No. It was transcribed as "Weiss", and all newpaper references to him before and after this give his surname as "Weiss". It's just the way it was written.
Hopefully that settles it. The surname of Weiss was always Weiss and was never Weifs.