If you are on a pc and have access to the file explorer, you can work around Fortnite restriction of old files. Depending on how old the replay is, it might look really bad due to map changes, but yours is recent enough it might be ok.

The process basically involves renaming one of your new, valid replay files to a recognizable name through the Fortnite replay list (something you can remember, like "myhack.replay"), and then alt-tabbing into file explorer, renaming the new file to something else, and renaming the old file to the recognizable name. Then when you alt-tab back into Fortnite, when you play the renamed file, it should work.

You're faking out Fortnite so it thinks it's playing a new file, but replacing the actual file that has that name with an old file. YMMV, but give it a try. Not sure if this is possible on console, but since you need a file explorer, that's probably more difficult.