Simple minibuffer completion question

[bmaron2]

Hello folks,

I've always wondered how to do this for ages now, and I'm hopeful
there is a simple solution which I haven't figured out.

Lets say you're editing a file named reallylongfilename.foo. You want
to save a copy and call it reallylongfilename2.foo.

If I do a C-x C-w, emacs prompts me for the new file name. Now, how
can I get it to insert the default file name (reallylongfilename.foo),
but still allow me to edit the text? If i just press RET it submits
the default name, not allowing me to add the '2'. TAB and SPC give me
a list of all the files in the directory, so I still have to type out
reallylongfilename.foo.

An analogous question is if I'm editing reallylongfilename.foo and I
want to open reallylongfilename.bar. How can I do that without typing
manually all of reallylongfilename?

Thanks in advance!