Wouldn't there be an equivalent '18' marker in the 18 library?
The marker object will show as a red dot when you open an old file but use the 18 library (unless you use the migration libraries - but I am not sure about these as I don't use the default libraries at all).
Select them and swap them for the '18' marker - use CTRL+ALT to pass on any customised parameters if you need to.
Otherwise if you really want to keep using the 15 marker object the easiest way will be to copy it from your 15 file and paste it to your 18 file.
This should add the object to the 'Embedded' library which is fine for that job.
In the Library Manager you can then export that library part to an 'office' library that you can load in all of your 18 jobs.
You can also 'Expand' or 'Extract' the Library Container File (LCF) for your 15 library allowing you to find the object file (via Explorer in Windows or equivalent on Mac) and copy it to your office library.
I would not place it in the "object library 18" as if this ever gets updated you will lose the 15 object - plus it too is an LCF file that you will have to extract to be able to place anything inside it anyway.
Barry.
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