[Copying slides from one presentation to another]

yes, & thank you for expressing what obviously I was unable to so do.

       There's also a problem with their copy & paste suggestion, since
that surely requires having more than one copy of LO open at one time,
which seems impossible since when I click on LO again, the opened one
appears rather than a new version.

       And when I've gone to the LO archives to attempt to download the
previous versions, it merely sends me back to the download page to download
the newest version.

         PLEASE EXPLAIN HOW to download a previous version, if the latest
versions will no longer have this option available.

With the latest versions of LibreOffice you have two different options
for inserting slides from one presentation into another one, both tested
and working on every platform.

A. You can open two different LibreOffice presentations at the same
time, and switch between them using the Window menu.

When you have two presentations open you can copy and paste slides
between them from the Slide Sorter view:

1. Select multiple slides with the mouse while keeping the Ctrl key pressed
2. Copying them by selecting Edit > Copy or by hitting Crtr+C
3. Switch to the other presentation from Window > Presentation Name
4. Past the slides after the selected slide from Edit > Paste or by
hitting Ctrl+V

B. You can insert multiple slides from another presentation into the
open presentation from Slide > Insert Slide from File...

1. Search and open the source presentation
2. Select any number of slides or the entire presentation
3. Click OK and wait for the process to complete

I have just copied/pasted according to option A and inserted according
to option B 60 slides of an 82 slide presentation into another 142 slide
presentation without any issue.

Although I don't see the need of reverting to an old LibreOffice version
to do the same thing in a different way, you can find old LibreOffice
releases here:

https://downloadarchive.documentfoundation.org/libreoffice/old/

Hi Anne and all:

As  I understood you are working with one Presentation file that is in cloud and the other that you need copy too.

Why not try to use two tabs in web browser to open each Presentation file, or use two different web browser to open each file, supposed that you can't download them and then upload them again. I don't know if these suggestions work but nothing to lose trying.

Regards,

Jorge Rodríguez