crissrudd4554 said:

Jaws
Terminator
Halloween
Psycho
Mad Max

There’s also been examples such as Grease and Scarface where although the original mixes were not mono they still were given 5.1 remixes later on that completely revised everything.

though fortunately for at least most of these movies the original mono/dolby mix was provided alongside the 5.1 version on their blu ray releases (Terminator being the only exception), though for Grease the original Dolby mix finally resurfaced in 5.1 form on the recent 40th anniversary release