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You can use `-Suw` for that. I haven't actually tested it, but it should work according to the documentation. From `man pacman` (redactied/edited for clarity):

> UPGRADE OPTIONS (APPLY TO -S AND -U)
>
>  **-w, --downloadonly**
>
> Retrieve all packages from the server, but do not install/upgrade anything.
> 
> [. . .]
>
> SYNC OPTIONS (APPLY TO -S)
>
> [. . .]
>
>**-u, --sysupgrade**
>
> Upgrades all packages that are out-of-date. Each currently-installed package will be examined and upgraded if a newer package exists. A report of all packages to upgrade will be presented, and the operation will not proceed without user confirmation. Dependencies are automatically resolved at this level and will be installed/upgraded if
necessary.

That said, it might be simpler and more practical to instead upgrade on a regular basis so you never have enormous amounts of packages to download.