replace front derailleur
 

I want to replace my front derailleur, and question is my chain is fairly new less than 500 miles, ( shimano ultegra chain ), can I unlink the chain, replace front derailleur and connect the chain again? do I need to get new chain link or something? thanks

Modern chains made for more than 8 speeds, including Ultegra, cannot be cut and spliced by pushing a pin out and back in.

This isn't to say it cannot be done, and many do successfully. However, these narrow chains have the plates sitting at the end of the pin, and depend on the pin being peeled over to keep the plate from migrating out and off the end. When you push a plate out, you break off those peeled edges, and lose the resistance to migration.

So, it's a made for the job splicing pin, or a connecting link.

Many front derailleurs (but not all) have a screw that can be removed so that you can pry it apart to remove the chain. That's what I do when I can.

Install a quick link on the chain, if you're opening it up. Conversely, front derailleurs commonly have a screw at the end of the cage, that you can remove; this would allow you to slide the chain in, without cutting the chain.

This is exactly why I quit buying Shimano chains & started buying KMC quick link chains. Get a Park Tool quick link tool. No more worries.

I don't see the difference. Shimano chains can also be closed with KMC quick links. Of course, there is a difference if links can be reused, but more and more this isn't the case, and the "use only once" rule for links may become universal with the narrower inks.

Depending on the FD, and the OP's mechanical skill, he might grind out the FD cage closure, and replace it with either a pop rivet or screw and nut.

IMO - one of the worst things that ever happened to FDs was the move to closing the cage with a rivet rather than screw and nut.