auto upper/lower in replace pattern based on search pattern?

[Marshall Abrams]

Hate to ask this because I'm sure the answer is already out there 
somewhere, but I've searched help and vim.org until I was blue in the 
fingers....  I could swear that I remember from a few years back that 
there's a way to do this.  Can't remember whether it required something 
special in the patterns or some special :set variable or loading a 
plugin.

I want to the replace pattern in a substitution to be able to figure 
out whether I want an upper or lower character in the replacement, 
depending on whether the search pattern matched an upper or lower 
character.  Very useful for substitutions on studlyCaps.

For example, suppose I want to replace all instances of 'first' with 
'last' and of 'First' with 'Last'.  Can I do that with a slinge :s/// 
line?

Thanks-
Marshall