Future version files appear in read-only mode when opened in the previous release. So you cannot edit SOLIDWORKS 2016 parts or assemblies in SOLIDWORKS 2015 SP5.0. However, you can use SOLIDWORKS 2016 parts and assemblies in 2015 drawings and assemblies. You can add new features to the files. These new features will appear below "Future version file." If you save the file and then open it SOLIDWORKS 2016, all the FeatureManager design tree data is available including your saved changes.
You can open SOLIDWORKS 2016 parts and assemblies using SOLIDWORK 2015 Service Pack 5. This will be the same for future releases as well. But... Backwards compatibility is only supported between consecutive releases. So, you will not be able to open a SolidWorks 2017 file in SolidWorks 2015 SP5.
Further, here are some things that YOU CAN and CANNOT do in SOLIDWORKS 2015 SP5.0:
1. Once a SOLIDWORKS 2016 part or assembly file is open in SOLIDWORKS 2015 SP5.0, YOU CAN view configurations, use the Measure tool, view Mass Properties and Custom Properties, and view Materials.
2. YOU CAN use SOLIDWORKS 2016 parts and assemblies in 2015 drawings and assemblies. YOU CANNOT edit the SOLIDWORKS 2016 part or assembly in SOLIDWORKS 2015 SP5.0.
3. YOU CAN work with SOLIDWORKS 2016 parts and assemblies as components of an assembly in SOLIDWORKS 2015 SP5.0.
In a SOLIDWORKS 2015 SP5.0 assembly, YOU CAN:
•Mate the 2016 component or subassembly to other components.
•Add a Bill of Materials that includes 2016 and 2015 components.
•Suppress and unsuppress 2016 components.
•Create a drawing of the assembly that includes both 2016 and 2015 components.
•Use interference detection when mating 2016 and 2015 components.
•Mirror and pattern 2016 components.
•See and reference default planes and origins of the 2016 part or subassembly.
In multi-configuration models, for each configuration that you want to access in the SOLIDWORKS 2015 SP5.0, configuration data must first be generated and saved in SOLIDWORKS 2016.
4. YOU CANNOT open SOLIDWORKS 2016 drawings in SOLIDWORKS 2015 SP5.0.
5. YOU CAN create drawings of SOLIDWORKS 2016 parts and assemblies in SOLIDWORKS 2015 SP5.0 with some limitations. YOU CANNOT insert model items or access FeatureManager design tree data such as weldment cut lists and weld beads.
6. YOU CAN create drawings of assemblies that contain both 2015 and 2016 components.
In conclusion, SOLIDWORKS has tried very hard to provide the functionality that its customers request. SOLIDWORKS 2016 does allow backwards compatibility with SOLIDWORKS 2015 SP5.0. This feature certainly has its limitations. It is still going to be up to the individual user and company to decide when the best time to upgrade is. Hopefully, though, this will help everyone decide to upgrade much sooner and take advantage of the latest and greatest version of SOLIDWORKS.