As Chris D already pointed out, it normally should not be a problem to rebuild thumbs via browser.
Though, rebuilding a large amount (say serveral 10 or 100K) thumbnails via browser is painfully sloooow as there is a significant overhead for strating each (tiny) batch.
Those who want and can use CLI might want to try the attached Add-on.
The commands als xf-rebuild:kirby-xfmg-album-thumbnails and xf-rebuild:kirby-xfmg-media-thumbnails.
Keep in mind that the command must be executed as the same user that runs XF via webserver (if you are using the default filesystem adapter, might be less important if using other adapters).
(To be 100% precise, it might be possible to use another user but those wo have setup ACLs will know that they are doing )
This is a really simple Add-on and I don't think there is anything that could significantly go wrong, but anyway: No guarantees whatsoever, use at your own risk and take a backup!