Does a security standard need to be applied again if the project is modified?

If you have applied a security standard to your threat model and then proceed to add new components to that diagram the standard will not apply to the newly added components by default.

In order for the standards to apply to the newly added components you will need to once again apply the relevant standard to your diagram for it to be included against your newly added components.

If you do not want the standard to apply to the newly added objects, then no action is required.

Related:

- How to apply a standard to all threat models