How to merge files from old OS to new OS on the same SSD

Nov 26, 2018
I recently upgraded my PC, but did not replace the SSD that has windows 7 and a bunch of games/files on it. After replacing processor and CPU, I installed a separate Windows 10 OS on that same SSD. When I boot up the new windows 10, all the previous files show up and I still have the same amount of space available as on old OS, but the folders are all empty. If I say, reinstall a program thats 10 gb, will it take up additional 10 gb or can the files in the previous OS be used? If not, is there any way to transfer files from previous OS and delete windows 7?
Programs from W7 disk will not work any more and will have to be reinstalled but any other data you can just copy to appropriate folders in new disk. After you transfer all personal files, you can delete all partitions on old disk, format it and use for storage.