Very good! But I have one more.
I use "Blend" map for that in "Multiply" mode.
1. Replicate "Base Color" ("Diffuse") map
2. Make necessary changes to replicated map
3. Apply that map to "Blend" channel (make sure the "Blend Method" is "Multiply") and set "Strength" to 0
4. On the timeline, where change should occur, set "Base Color" ("Diffuse") "Strength" to 0 and "Blend" "Strength" to 100
5. Set transition curve to "Step" for the key.
Smooth transition is still not possible however, so the "Step" must be used.
I just wish the "Blend" channel would eventually provide a full support for 24 bit PNG files (for alpha channel particularly).
Then there would be endless possibilities for smooth transition adding all kind of "spotty" effects to the "Base Color" map without distorting it overall.