There is no east way to restore cache to the normal files. It is explained in
UnrealReference.
The best option is indeed to keep your cache files forever. Open Unreal.ini, go to the option PurgeCacheDays= (under[Core.System]) and change it to PurgeCacheDays=0. 0 means: never purge cache.
I advise anyone to handle it's cache like gold, and backup it from time to time (copy it to an other folder or burn it on CD) In this way, your cache folder may grow large, but after a while you can join the most servers without downloading a single file.
(I'm building my cache since 1999, and it already 3 gigs large. This may be an exception, but if you play much online the cache folder will grow to several hundred MB's.)