Apple changed all sorts of shortcuts, but to switch between header and implementation, the new shortcut is,
⌃ + ⌘ + ↑/↓
You can change it back to the previous ⌥ + ⌘ + ↑/↓ in
Look for the Jump to Next Counterpart and Jump to Previous Counterpartcommands.
If you are working on a big screen, you could use the Assistant layout's split view, and have the .h and .m files side by side.