Agents who have upgraded Heroes to the maximum amount will continue to pull maxed cards from capsules, but don't get your time traveling pants in a knot! Your extra cards will be converted to XP. Here's what you can expect:

Common Cards: 5 XP per extra card
Rare Cards: 50 XP per extra card
Legendary Cards: 500 XP per extra card
Epic Cards: 1,000 XP per extra card
Historic Cards: 20,000 XP per extra card