It depends more on which packages you use in Debian .
In general, this situation is very possible. Rather theoretically, but still.
If you use Debian 8 and rely heavily on new packages, after migrating to Debian 7 you may find that there are no packages in the official repositories. Totally.
Most likely, they will be able to find a replacement. Most likely, it will not even be very difficult. But you may need to update and dependencies. Collect something from the source. In general, the “worst case” may involve many adventures, but it has little chance.
Your doubts would be understandable if your development system was on Debian 8 and it would be a pity for you not to work on it. But if you are still going to install the system in VirtualBox, then it is better to install the same version of the OS that you expect on the server. So expectations will be much better docked with reality.
Oh yeah, the most important thing.
Debian has an archive of images of old versions . You can get the version you need there.