Refer to IRC transcripts here for additional info:
https://lists.gnucash.org/logs/2017/12/09.html#T15:08:18

If the account renumbering tool is used, the tool's string treatment of account causes renumbering issues as follows:

Case 1: Without leading zeros:
Old account number --> new account number
01-10-1 --> 01-10-1
01-10-11 --> 01-10-2
01-10-2 --> 01-10-3

Case 2: With leading zeros:
Old account number --> new account number
01-10-01 --> 01-10-1
01-10-02 --> 01-10-2
01-10-11 --> 01-10-3

The current implementation of account renumbering will always produce case 1, as the tool currently does not insert leading zeros.  User needs to manually insert leading zeros for the renumbering tool to properly pickup on the account numbers, which at that point, the user may as well manually renumber everything.

The feature request: default the renumbering tool to always insert leading zeros, or at least provide this as a checkbox option on the dialogue box.  Leading zeros will allow all future renumberings to proceed correctly, without need to get the renumbering tool to be able to parse out the integers (the renumbering tool can remain treating everything as string).