If you have all your primarch slots currently occupied by summoned primarchs you can swap one of them out for a different primarch type by the following steps:
- Select the primarch you want to replace
- Move that primarch to your Home
- Tap the primarchs button and then tap the “Tech Tree” option.
- Select the primarch you want to summon from the tech tree and click “Summon”
- This will replace the primarch that you currently have selected at your home with the primarch type that you just selected to summon.
- The troops that were on the primarch you're replacing will be swapped to the primarch that is being summoned once summoning is complete. You cannot Replace a primarch with more troops than the new one you're trying to summon in.
- A primarch that is being summoned in cannot be attacked.