No need to re-join.
Select (highlight) all the clips that you want to effect.
Click an FX icon on any one of those clips.
Add the effect. If it doesn't immediately apply to all selected clips click the "replace effect chain..." box icon with the green check mark on the toolbar. 
Any time you make a mistake immediately press <ctrl-Z> to go back, step-by-step.
Save Project File AS routinely, giving it a unique # or name, so as not to overwrite prior saves. Using Save Project Files (no AS) will overwrite. From the HOME tab, that choice is available by clicking the chevron to the right of the Save Project button on the toolbar. Then you're protected and can use the VPJ to open the saved project.