This is what the Help function in Excels says !!!

Microsoft Excel won't merge the workbooks I want to combine.
Requirements for merging changes To merge the changes from one
workbook into another workbook, both workbooks must meet the following
requirements:

Both workbooks must be copies of the same workbook.

Each copy must have a different file name.

The workbooks either must not have passwords or must both have the same
password.

The workbook from which the copies were originally made must have been
a shared workbook.

When the copies were made, the change history must have been turned on.

At the time that you merge the copies, the change history must have
been maintained in both copies for at least as many days as have
elapsed since the copies were originally made. For example, if the
change history is being maintained in the workbooks for 30 days, and
the copies were made 45 days ago, you can no longer merge the
workbooks.
When you turn on the change history but aren't sure when you'll merge
the workbooks, you can preserve the change history by setting a large
number of days to maintain the change history, up to the maximum of
32,767 days.

Merge changes manually If Microsoft Excel cannot merge the two
workbooks, you can incorporate the information from one workbook into
the other manually, by copying and pasting the information from one
workbook to another.



best regards
Gert