It was stated that it was done as a precaution. What is the reason for the precaution? Probably two reasons, one technical, the other political. The technical reason is that if there is material shifting around, it can change the geometry in ways that are not fully characterized. One of the principles of criticality safety is to go the extra mile to assure that you have inserted negative reactivity to offset any possible increase in reactivity as a result of the geometry change. The political reason is that if they didn't do it, the anti-nuke kooks would accuse them of not taking proper precautions.
Once the new geometry has stabilized, they will likely reduce the boric acid concentration gradually and check to be sure criticality is not being approached. That is a very straightforward reactor physics measurement involving subcritical multiplication. One of the classes I teach does the very same measurement, not by boron concentration but by control rod positions. It is called incremental approach to critical. You get extremely precise measurements of critical rod position. Same deal with born concentration.
Can understand that, but it has been reported that they ended up adding 1,094 Kg of boric acid. More then they originally estimated would be required. That does not seem to be a well calculated preventative maintenance procedure done partially for show.