In 2007 I started a thread on the book translated as "Spirit of Aikido".
One of the main assertions made is that OSensei changed the meaning of "Aiki" thus creating a new art.
The thread is here.
I would assert that his son and many followers definitely changed the definition of "aiki" to be something new and different than how it was used in Daito Ryu. I doubt strongly that OSensei actually changed what he was doing at all.
I try to use the term as it was used, not as the Aikikai, Tohei or the various other modern lines seem to use it (which according to the book was to join ones energy with that of the attacker and the universe). I'll note too that my definition of "Aiki" has changed a lot since I started that thread, but the idea that there was a shift in the meaning of the term seems fairly relevant.