Originally Posted by bostontrevor

No, new components need new components.

New components will "wear" to old components. Put a new cog on an old chain and that stretched chain will ramp the cog until it fits. It's meshed now, but it's not so innocuous as that word might imply.

And of course the reverse is true if you have a new chain on old cogs or chainrings.

It the meantime, untile they've mated to one another you can have some seriously sloppy chain/tooth interfaces which put you at risk of launching your chain. (A badly worn drivetrain is also at risk of this, but a mismatched drivetrain is the worst)