Appendix G of Sather's excellent Understanding the Apple IIe has a summary of the changes:
The primary operational improvement of Revision B is the addition of
the DOUBLE-RES graphics capability. This was achieved by rewiring 1/3
of the C5 LS10 NAND gate so that pulling pin 55 of the auxiliary slot
low forces TEXT processing at the timing HAL instead of disabling
motherboard ROM as it did in Revision A. [...] The nomenclature of pin
55 was appropriately changed from ENFIRM to FRCTXT'.
Changes in the timing HAL and MMU were required to make the new
FRCTXT' wiring work. The Revision B timing HAL treats pin 12 as GR+2'
whereas the Revision A timing HAL treats pin 12 as GR+2. The MMU was
changed so that ROMEN1' and ROMEN2' are gated by INHIBIT', a
necessity since the ROM inhibiting function had been performed by the
NAND gate which now performs the forced text function. [...]
Wiring and component changes were made to the video summing amplifier
in Revision B, and these would be necessary to realign COLOR
REFERENCE to the PICTURE signal if video timing in the HAL were
Another area of change in Revision B was in the functions of
motherboard configuration jumper pads. Two new jumpers were added (X3
and X7), and the functions of XI and X2 were changed.
According to Apple II History the feature was designed after Revision A release:
Early Apple IIe motherboards were labelled as “Revision A”. Engineers
determined soon after its introduction that if the same use of
parallel memory was applied to the hi-res graphics display as was done
with the text display, they could create higher density graphics.
These graphics, which they called “double hi-res”, also had the
capability of displaying a wider range of colors, similar to those
available with the original Apple II lo-res graphics. The IIe
motherboards with the necessary modifications to display these double
hi-res graphics were labelled “Revision B”, and a softswitch was
assigned to turn on and off the new graphics mode.