The IBM 3270 DID have selective updating. If "Forms" were used (i.e. BMS mapping under CICS for example) that only worked on READ modified commands but (natively), any byte on the 1920 byte screen buffer could be updated independently.
When the ENTER key (or PF key) was pressed, only the modified data was read - preceded by a 3 byte "set buffer address" (SBA) order (to identify the field on the screen).
The read buffer command was only used mainly for debugging purposes and was actually used in OLIVER - my own developed CICS test/debugging System that was used by more than 600 blue chip corporations in the 1970's and through to the 90's.
I have recently posted some early promotional material about OLIVER and its companion product SIMON to Google+ collection. It was by the way never called "IBM OLIVER" - that was the result of some Wikipedian deciding to prefix it with "IBM" (dont ask me why!)Here is a "Google sheets" document discussing the WORKS RECORDS SYSTEM (that OLIVER was used to create in the early 1970's at ICI)
https://docs.google.com/spreadsheets/d/19mi9x31pA5u4-GkYrj1gT45Rd18t4vqODgtMrIKAP4E/edit#gid=1710083428
and below are various links to the publications mentioned above about OLIVER and SIMON:-
https://plus.google.com/u/1/collection/wUwasB
https://plus.google.com/u/1/collection/oXlasB
https://plus.google.com/u/1/collection/cDbasB
https://plus.google.com/u/1/collection/Mk1asB
https://plus.google.com/u/1/collection/8vIZcB