Libero IDE cannot determine if the structural module in your old design used a synthesized netlist or operated directly on the source files.
If your design was created from an EDIF netlist then you do not need to skip synthesis and you may proceed.
If your design was created from the source files, you must configure your Libero flow to skip synthesis. This automatically creates a new implementation. You may then copy the ADB file from your old implementation to the new one in Windows Explorer. The updated ADB file is immediately available in the Libero IDE File Manager after you copy it into your new implementation You may run synthesis as usual after you copy your ADB file.