Does the upgrade destroy the existing resonator or is the old one swapped into your inventory?
I found that upgrading makes the resonator that was there effectively disappear; It doesn't get dropped, it doesn't go back into your inventory. It is no more.
However you can save yourself some trouble by not pressing the deploy button or swipe action. Select the portal, select upgrade portal, scroll through the empty slots for resonators, and on each slot scroll through your resonator inventory exactly specifying the level you want to put in that slot. No need to upgrade over an existing one unless you're actually looking at a portal you didn't just capture (say someone on your faction put down a bunch of L1 resonators) and want to upgrade.
Upgrading a resonator will replace it with a new one, effectively removing the old one from world.
Upgraded resonator will have full energy, regardless the conditions of former resonator.
Note that you cannot upgrade a resonator if the portal isn't fully deployed.
Several things happen:
- The original owner won't get notified. Unlike when it is destroyed or flipped.
- The limitations of resonators per agent will get recalculated. This means that when your resonator is replaced, you can deploy another one. Most efficient way to upgrade a portal
- If the upgrade increases level of the portal, it will produce new XM within 2 minutes.
But most importantly:
- If your last resonator is replaced on a portal you own (captured), you LOSE the claim for Guardian badge! You won't get any new days towards this badge!
However, you can place new resonator and you will gain full days on next tick.