Old Time Machine Backups not accessible with new Mac
I've changed from a 2008 Mac Pro (El Capitan) to an iMac (Mojave). The TM backup was on a external HD which I connected to the new Mac telling it this was the HD it had to use for TM. Unfortunately I cannot access anymore the backups prior to the computer replacement: the older dates in the timeline are gray-red instead bright and nothing happens. The old backups are still there though, I can see them in Finder. Thanks for any ideas how to "merge"the old with the new backups. Christoph
iMac 27", macOS 10.14