Does Experimental Hexplate add the Mech tag to existing tags, or does it replace them? Does the card work given that 'I am a' is not explicitly listed in the rules?

Ruling: Experimental Hexplate adds the Mech tag without removing existing tags. The phrase "I am a" functions the same as "is" and "are" (all conjugations of "to be"), which the rules cover.

Nuances:

• Cards that affect tags work additively unless the card explicitly states otherwise
• The rules example stating "Other friendly units are Yordles" adds the Yordle tag rather than replacing existing tags, which establishes the additive principle
• No existing tags are lost unless a card specifically says so