Versions of LabVIEW later than the code you open them in will automatically upconvert them. There can be issues if your old code uses packages that have been substantially updated (like Traditional DAQ to DAQmx).
Just be sure you have backed up (or use Version Control!) to keep the old copy safe until you're sure the upconvert was successful. Note that if you save the new file LabVIEW will NOT prompt you to choose a new name and will overwrite the old version!
There is also a Version Conversion board to help you. Especially if you need to downconvert past the File > Save for Previous Version... option.
It's also a good idea to search for "Labview Compatibility" to see the charts that show you which versions of LabVIEW, Window, DAQ, etc. can work with each other.
According to that chart, you can open VIs saved as far back as LabVIEW 6.0. But as already stated, you will likely have other issues due to drivers (Traditional DAQ vs. DAQmx likely being the biggest issue) and so many libraries that have changed over the 15+ years (7.1 to 8.0 had some MAJOR changes).