I've looked around, and it seems to depend on the stylesheet of use you learn with.
Ones that do say that "only additional words [beyond the original quote] should be enclosed", rather than indications of substitution, seem also to be happy with substitution for case or tense rectification. "[A]dditional words" clearly isn't being applied consistently here, nor "words [that] should [have been] enclosed." But there are always exceptions, it seems, so no harm.
Others, however, are fully enthusiastic about direct substitution (including bracketted ellipses to show an edited-omission), and the standard for edited
additions is to be explicit about it: "[…] additional words should be enclosed [
Editor: but then I must always credit my insertions]."
Noting that in most of my writings I've abandoned the strict rules for direct speech hammered into me back in the days of fountain-pens ("{<Any intro text> comma space} open-quotes <Speech with starting capital> foo-mark close-quotes {space <any outro text.>" Where 'foo-mark' is: always exclamation-/question-mark if part of the Speech; otherwise the Speech's original full-stop(/period) if there's no outro; elsewise converted to a comma if there is) as far too pernickety, and web rendering has largely stopped me double-spacebarring between typed sentences. Yet I find a lot of my learnt methods still far too logical to switch just because Chicago has a different idea from AP, or both disagree with the Times ("of London"!) preference. Including my using of "-ise", "-our" and "-tre" British endings even in here, over the more Websterised anglic. (I did
deliberately use "center" in my "Power Cable, Montana" quote, above, just for proper verisimilitude. In fact, it distracted me so much that I wrote that instead of "…, Nebraska"! I hope you're happy about that...
)
In other aspects of typography, personally I abhor the Oxford Comma - best to rewrite, if the OC would 'make it clearer', perhaps with a semi-colon delimitered 'greater list' - and don't talk to me about pluralising initialisms with apostrophes[1]!
YMMV, naturally.
[1] Or anything. Is our very own Forum Games's[2] "Cabal of Villain's" thread clearly just demonstrating the extent of its villainousnesses by the very title?
[2] Pozsessive of a proper name that is
not biblical, before you ask...