HIPEs that are not on the brink of changing status are discussed through Github Issues. We generally use Issues to discuss changes that are controversial, and PRs to propose changes that are vetted. This keeps the PR backlog small.
Any community member can open an issue; specify the HIPE number in the issue title so the relationship is clear. For example, to open an issue on HIPE 0025, an appropriate title for the issue might be:
HIPE 0025: Need better diagram in Reference section
When the community feels that it's reasonable to suggest a formal status change for a HIPE, best efforts are made to resolve all open issues against it. Then a PR is raised against the HIPE's main README.md, where the status field in the header is updated. Discussion about the status change typically takes place in the comment stream for the PR, with issues being reserved for non-status-change topics.