Here is a way to do it at once without any script, through SQL.
Do this in the SQL Scratchpad in EA.

Copy the GUID of the target package in the clipboard, say {61068DDF-335B-4b29-89F8-C54A7000EF6F})

Its package id is the following:
select Package_ID from t_package where ea_guid='{61068DDF-335B-4b29-89F8-C54A7000EF6F}'
Copy the GUID of the source package in the clipboard, say {F3B52DA0-D9E9-4606-BCA0-5D211B73FBE6}.
Its package id is the following:
select Package_ID from t_package where ea_guid='{F3B52DA0-D9E9-4606-BCA0-5D211B73FBE6}'
So, elements in source package are:
select * from t_object where Package_ID=(select Package_ID from t_package where ea_guid='{F3B52DA0-D9E9-4606-BCA0-5D211B73FBE6}')
So, let's make that go to the target.
update t_object
set Package_ID=(select Package_ID from t_package where ea_guid='{61068DDF-335B-4b29-89F8-C54A7000EF6F}'
where Package_ID=(
select Package_ID from t_package where ea_guid='{F3B52DA0-D9E9-4606-BCA0-5D211B73FBE6}'
)
Should do the trick.
Backup your model first.
For an Add-In, it is just a matter of getting the two GUIDs, and Repository.Execute(theSQLString).
Repository.Execute is not in the doc, but works.