I cannot reproduce your issue. For me, @attach does replace attachments on the same points as the ones worn from the RLV folder...
Please, verify that your already worn attachments are not locked (in which case, they won't detach, as expected). Use the "Advanced" -> "RestrainedLove" -> "Restrictions and commands log" floater to track all the commands and restrictions.
Note however that @attach won't replace worn attachments when the folder you force-attach got a "+" sign as the first character of its name. Note also a small incoherency in Marine's API documentation, since the phrase you cite is actually deprecated as shows the documentation for "@attachoverorreplace":
Meaning the "If you want to give the user the choice through the name of the folder (as indicated above, by prepending the name by a "+" sign by default), use @attachoverorreplace" part of her note for @attach is deprecated/misleading as of RLV v2.5, when @attach was made to behave just like @attachoverorreplace; the API evolved with time... and in any case, the code for @attach* is in sync in both Marine's and my viewers.