changed OS, time machine won't access old backups

I have learned a lot about this problem in the past few days, as I have struggled to move data off of a time machine backup that the Migration Assistant would not recognize.

The reason you are seeing the little red dots (stop signs) is that you do not have the proper permissions to read these files. I have learned of two ways to get around this problem.

The first way is to transfer the files to a temporary location on your hard drive, repair the permissions, and then transfer them to their desired folders. To repair the permissions you need to right click on the folder, Get Info, then in Sharing and Permissions open the lock and allow full Read/Write access to your User. Then go to Disk Utility, First Aid, and "Repair Permissions" on your hard drive. Once it does its work the files should be unlocked for your User, and you can put them where you want.

Another way which I found easier and involves less steps is to set up and log in as a "Root" User, which makes you a "super User" and gives you Permissions to do anything you want with any file. Obviously you can get into trouble in this mode, so only do it if you have some confidence that you won't mess anything up. When logged in as Root, you can move these files freely and put them where you want.

I hope this helps. This encapsulates about 5 hours of my own learning yesterday in dealing with the same sort of problem.