How about changing the copybook in the middle of hard pen calligraphy?

It depends. If you practice well and are almost unknown, just change; If you haven't practiced yet, changing it may have an impact.

Wang Xizhi's Preface to the Lanting Pavilion is quite good and beautifully written, but all of them are copied. Yan Zhenqing's "Sacrifice to the Nephew" is also very good, but it is difficult and can be practiced for a lifetime; Zhao Mengfu's is good, too. Generally speaking, irregular handwriting will not affect the number of papers marked by computer, as long as you pay a little attention and write correctly.