Given the stroke order and direction is the same between a brush and a pen, there wouldn't have been significant amount of "re-learning" that would need to be done from a *mechanical* perspective. I couldn't find any first-hand accounts of anyone struggling from that side of things. Non-brush forms of writing, such as with charcoal or carving, were already well-known, so the concept would not have been that foreign.

One thing of import is that it's very difficult to do the various forms of kuzushiji, or highly stylized, individualized cursive without a brush. This kind of abbreviated writing was very common up until Meiji, and slowly became less common once people started switching to pens. The types of kuzushiji that are practical with a pen or pencil are different, and would have been slightly new (but not incomprehensible) to someone seeing them for the first time.

Keep in mind that there were much larger, more disruptive changes to the Japanese writing system that occurred around that time, which would have been much more top of mind for people and harder to formally learn:

The written language was being unified by government edict around its choices for which shortened form of which kanji would be used for which kana. Simplified forms of kanji were continuing to be standardized. There was also a short-lived and ultimately unsuccessful attempt to make Japanese adopt spaces between words like Western writing systems. Disagreements about when to use hiragana v. katakana persisted even until the 1960s!