Pretty much every change to the language needs an RFC.
Language RFCs are managed by the language sub-team, and tagged T-lang. The
language sub-team will do an initial triage of new PRs within a week of
submission. The result of triage will either be that the PR is assigned to a
member of the sub-team for shepherding, the PR is closed as postponed because
the subteam believe it might be a good idea, but is not currently aligned with
Rust's priorities, or the PR is closed because the sub-team feel it should
clearly not be done and further discussion is not necessary. In the latter two
cases, the sub-team will give a detailed explanation. We'll follow the standard
procedure for shepherding, final comment period, etc.
Sometimes in the implementation of an RFC, changes are required. In general these don't require an RFC as long as they are very minor and in the spirit of the accepted RFC (essentially bug fixes). In this case implementers should submit an RFC PR which amends the accepted RFC with the new details. Although the RFC repository is not intended as a reference manual, it is preferred that RFCs do reflect what was actually implemented. Amendment RFCs will go through the same process as regular RFCs, but should be less controversial and thus should move more quickly.
When a change is more dramatic, it is better to create a new RFC. The RFC should be standalone and reference the original, rather than modifying the existing RFC. You should add a comment to the original RFC with referencing the new RFC as part of the PR.
Obviously there is some scope for judgment here. As a guideline, if a change affects more than one part of the RFC (i.e., is a non-local change), affects the applicability of the RFC to its motivating use cases, or there are multiple possible new solutions, then the feature is probably not 'minor' and should get a new RFC.