Well first, the symbol scaling was never engineered to be changing willy-nilly like that project to project.
You should have two Preference Sets, one at 100%, and the other at 70%, and use them for the projects as necessary.
What you are witnessing, is the issue of the block definition of sprays. Once a block is defined, the system will use that rather than recreate it every time.
But other tools like Copy Along Line will not stop to check for if the symbol scaling has changed, they will just copy the block as is.
Overall, changing the symbol scaling halfway through a project would be the same as changing the scale. You're going to have a mess of things.