I know this may be “Basic Knowledge” to a lot of you, but I still see people struggling with this and screwing up their assemblies and drawings all the time because they don’t understand it.
The only IMPORTANT thing to know about this in SOLIDWORKS is that if you have an OPEN assembly or an OPEN drawing and you do a SAVE AS on one of the files that the assembly or drawing is referencing, it will make the assembly or drawing look at the NEW file instead of the old file !!
SAVE AS COPY will maintain the existing references, and just make a new file on the hard drive for that part.
Once you understand THAT, you will be MUCH happier in SolidWorks.