I'm interested in buying Mienecraft premium, but i've got pirate version of minecraft 1.5.2. I have played a lot on this version and i've got lots of mods and saves. I don't know if i could copy that to minecraft premium. From what I know on MC premium there's option to change versions, so maybe there's way to move my mods and saves to 1.5.2 version on premium. You can write me how it exactly works and whether after this action will be able to change the version to another without removing mods?
When you buy MC you can run pretty much any old version of it, IIRC. Any mods you use should continue working on same (1.5.2) version and same goes for saves.
Going beyond that, you would need to check if mods you use work for later versions. Back up your saves and try ooening them in new version - you MIGHT get worldgen glitches at your explored borders though.
Mods that you download for one version of Minecraft (1.5.2) will NOT work for any other version. You will have to download a version of that mod that is for the version of Minecraft you want to play in. Not all mods will have updated to the version you wish to play in, and in those cases if there's no similar mod to replace it then you are completely out of luck.