A Disclaimer: This is kinda hacky, and I don't recommend doing this as a rule. If we ever shift the phase or priority of the eval rule, it might break the following workaround.
It looks like the eval rule runs in the Validation Phase, at Priority 20000. So, make your eval script which deletes the tag (the one ShadowChemosh helped you with) happen shortly before that. Say Validation 19900.
Now try adding a second eval that runs shortly after the eval rule, say at Validation 20100. You should have a handle on deleting tags from the example above, and assigning them works much the same. It uses the "assign" command, like so:
Code:
~ Add dead god tag to the hero
perform hero.assign[DeityCat.DeadGod]
Give that a try, and hopefully that'll get things working. I haven't really messed around or tested the above, but the theory is sound, I think.