This is extremely browser-dependent--I believe it is
possible with IE4+, but with Netscape it may only be
possible with NS6.
It requires a selection object which tracks the cursor
position and sets the selection equal to the cursor+1 char,
so that the next keystroke replaces the selected character.
In effect, this is very tricky to implement, since it
requires a keystroke processor (like an onkeydown or onkeyup
event-driven script) to understand all keystrokes so that
cursor movement, backspace and so on work as expected.
If you are using IE5, you may want to check the features on
the MS website, as there may be some new modes that can be
set for an entry field to make this automatic and easy--we
will try to check also for you.