Upgrading items w/o unused upgrade slot?
All items should be upgradeable, with the exception of the new “Ascended” tier gear. However, you will be overwriting the old upgrade. (So you’ll get rid of the 10% increased bleed duration for whatever else you’re getting)
You can have as many crests as you’d like of the same type in your gear. Keep in mind that you cannot apply upgrades to items if the upgrade is a higher level item (So a Crest(level 80) could not be put in a level 79 exotic.)
I have run gear with 8 crests of the soldier (6 on the body, 2 on the weapons) in the past, so it is possible.