or were the first 9 volumes republished with different covers (probably after it started serialization in Morning)?
Yes I'm answering myself
Well first from Reimu Manga
Young Magazine Uppers (Volumes 1-9)
Weekly Morning (Volumes 10-13+)
And then if you check Kodansha's shop site
Volume 1, 2 and 3: released on 14/4/2005
Volume 4, 5 and 6: released on 23/5/2005
Volume 7, 8 and 9: released on 23/6/2005
So its obvious its not a new manga, but the same manga republished with different covers (and under a different comic imprint).
Meaning the first 9 volumes of Piano no Mori manga (old covers and new covers) are the same manga :P the manga continued in Morning from where it stopped in Young Magazine Uppers.
In other words taking the manga that got serialized in Young Magazine Uppers (the 9 volumes), changing their covers and publishing them under a different comic imprint (Morning KC).
The manga that got serialized in Morning is as mentioned from volume 10 and above.