I know this is old, but you can litterally just delete every file but the songs, the skins, scores, and the osu file, and then it will update and recognize all the old stuff you imported, was getting rather annoyed and figured this out, osu will just replace all the files, Idk if its presence, the bat file, collection, logs or the notepad, or the user file, one of those lil basterds blocks osu from just accepting new songs like a good little beatmap game tho so delete all those bastards you dont care about and gl, it worked for me.
if you ever have difficulty with importing songs and reach this part in the thread feel free to dm me on discord im Crimm#2323
To specify the issue I figured out how to resolve this way is importing songs from another pc, and I solved it by deleting everything in the folder other then the osu launcher, and the songs and letting osu fix itself after I relaunched it, im assuming the same would work for skins that fail to import ext tho, but I didnt care enough to copy the one skin i had over