My 7-yr old pressure-treated deck is starting to look bad on the flat
surface. The boards are cupping, cracking and its getting to be more
difficult to stain it.
The vertical boards (the skirting, the rails, etc are fine). Its the
flat horizontal boards that are bad.

What would it take to replace the flat surfaces with trex? Would the
rails have to be dismantled/removed, or can the replacements be put in
place around them?

Is it a good idea to do this?

Also, I had used a solid dark cedar stain on my deck. So the rails
still have this color. If I get the dark trex, I am hoping this would
match the rails, so I don't have to restain the rails.

Any ideas/suggestions would help. Thanks.
Mark Sabatini