[AUCTeX] 3.1.5 Changing the font

[juan mari alberdi]

Hello,

Reading the manual, section 3.1.5, I've found

"If you want to change font attributes of existing text, mark it as a  
region, and then invoke the commands..."

but it doesn't seem to work that way, I mean, having unmarked text written  
in the LaTeX default type face, if I mark a region as in Emacs (C-space ->  
go to the end of the text -> M-w) and invoke AUCTeX font commands (C-c C-f  
C-... or C-u C-c C-f C-...) nothing happens. It seems that AUCTeX only  
changes text that AUCTeX itself has previously marked. For example,

\textit{amama(Emacs point is here)} -> (C-u C-c C-f C-r) -> \textrm{amama}

Am I right?

Another thing, in section 4.2 there is an explanation of the command  
"TeX-font ARG"

     (`C-c C-f') Insert template for font change command.

     If REPLACE is not nil, replace current font.  WHAT determines the
     font to use, as specified by `TeX-font-list'.

but I can't see what those REPLACE and WHAT refer to.
I can't find in which customization group this command is, and that  
difficulty in finding things happens to me with other commands/variables  
in AUCTeX and in Emacs in general.

--
Juan Maria Alberdi