I rearrange (Sigil split) books all the time with Sigil.
The links are adjusted properly
If you "Cut and Paste" parts that contain "Anchors", then Sigil does nothing.
If you move the position of a split in the Booklist, Sigil adjusts the reading order.
Easier to maintain
Rename the splits to something meaningful (and terse, so it does not scroll off the viewer). p01C04.xhtml