Copy assignments from one molecule to another

You can copy assignments from one molecule to another (the numbering of the common atoms must be the same) by selecting the first molecular structure with the assignments and doing 'Ctrl/Cmd+C'. Finally, select the second molecular structure and follow the menu 'Home/Paste/NMR assignments'.  Please bear in mind that the numbering of the common parts of both molecules must be the same.

Another way would be to copy&paste the molecular structure with the assignments and make the structural changes in one of the molecules. You could renumber the resulting molecules by selecting 'Concatenate Numbering' from the 'Molecular Drawing Toolbar':