Hi AnnieD.

I made some compatibility tests. 
Technically I can open, and I opened, scenes done in the version 7 with the version 6.3, without problems. 
As the Instance lab didn't implement new, mainly in the structure, difference that disables the opening for a previous version doesn't exist. 
But it is good to do what TheBryster suggested. 
The whole project of Brycetopia 2 that is composed of 5 kits is being made in the version 7. 
To each scene that I end, I open her/it in the version 6.3 and I make a new one to surrender and I save in the 6.3. 
The great difference that I noticed is due to the amount of objects and surrendering that is faster in the 7. 
Another problem in the 7 is the delay in the export of objects.