help with fix atom/swap command

Dear all,

I built a yttria-stabilized zirconia system and I want to swap yttrium with zirconium atoms from time to time. Yttrium has +3 charge and zirconium has +4. The fix atom/swap doesn’t allow to swap two types of atoms with different charges, am I right? Is there a way to do that?

Thank you!

Shenli Zhang