So my boss just asked me to verify whether or not the later Service Packs include the fixes from all the previous SP's.
I have no idea.
If I upgrade from SW15 SP3 to SW15 SP5 will all the fixes from SP4 be there?
Or should I make him step from SP3 to SP4 to SP5??
And if he finally accepts that he needs to be on SW16...
Is there any problem with going straight to SW16 SP3 from SW15 SP3?
What is the best method to upgrade and/or update SW?
Yes, your fixes will be there.
As for upgrading, I always do a clean install when going from one major release to the next. I think most users follow suit. Use your copy settings wizard to create a copy of your registry settings, then completely remove SolidWorks, including registry entries, from your machine. You'll also want to search your C: drive, including hidden folders, for any SolidWorks/Dassault folders and get rid of them as well. Obviously, any folders containing parts/assemblies/drawings should be left alone. Once done, install the new version. You can then use the copy settings wizard to restore your previous settings from the file you created.
If at all possible stop using any version of SolidWorks older than SW 2015 SP5. This is for the simple reason that they Windows installer update issue is fixed in this release. If you have Windows 7 or newer and have had the Windows Installer security update applied you have to run the "Run Before" and "Run After" scripts. From experience that doesn't always work (or should I say ---- the users don't always do that as instructed). Then, the SW installation is broken in some random fashion.
If I were you, I would either upgrade them to SW2015SP5 or SW2016SP3.
From the SolidWorks software download page in the customer portal there is a link at the top that will let you see what was fixed in each release.
That takes you to this page: http://www.solidworks.com/sw/support/subscription/showNDA.htm?&actionPage=/sw/support/subscription/ServicePackMaintenanc… (make sure you have an account and accept the NDA).
All previous fixes are included. Personally, I never upgrade from version to version. I always install the newest release side by side with the previous. Once I am confident that the new release is stable and doesn't have any show stopping problems, I may uninstall the previous release if I need to free up hard drive space.