Yes, the system as currently implemented does let you change secondary guests yourself online and even the lead/primary guest if you contact member services.
And while you are correct that there is nothing in the rules that says that you
can't change the names on a prebooked reservation, there is also
nothing in the rules that says that you are allowed to either. (Note: Other than a small section I found in the areas dedicated to favorite weeks, because they book those for you and you do not get to choose the guests at the time the reservation is made).
So they technically could stop allowing you to modify guests on a reservation online at any time (Though IMO they most likely won't ever take that off the online system because there would be a backlash after having it in the booking system for so long and a major increase in labor for people contacting MS for any changes)
But (as I have said before) there
are areas of the rules that specifically state that
a reservation being used as a rental must be communicated to DVC at the time the reservation is made. Not once you find a renter for it, not once you are ready to modify it, etc. Right when you make the reservation. If you do not follow that rule, they do not have to allow you to make the changes for a rental, even if they have in the past.
There are also clauses that say DVCs approval of guest(s) using the room instead of the member(s) is
not required once the reservation is made in the guest's name. Which means that they may decide that approval may be required if you are trying to change the lead guest on a reservation from the member to a different guest. So they could deny lead guest changes for possible rentals in that way as well if they wished.
All this to say that basically:
How the online system currently works or what they currently allow you to do via member services do not 100% mirror the actual rules for a variety of reasons. They may be lenient now/in the past, but they can also be more strict in some ways if they wanted to right now (or obviously create new rules and add them as well)
EX. The rules had always said that a member was allowed 1 transfer only. They were lenient and allowed members to do more than one transfer if it was between contracts that the same person was an owner on. Until they weren't lenient anymore and decided to enforce the letter of the rules (as well as thankfully adjusting the banking/borrowing the transfer rules to add some flexibility for members using their points and not stripping and flipping contracts)