If a creature I control is being targeted by an enchant creature my opponent controls, can I equip it with an artifact that grants hexproof to counter the existing enchant creature?

Example: My opponent casts [Kasmina's Transmutation](http://gatherer.wizards.com/Pages/Search/Default.aspx?name=%2b%5bKasmina%27s%5d%2b%5bTransmutation%5d) on my [Serra's Guardian](http://gatherer.wizards.com/Pages/Search/Default.aspx?name=%2b%5bSerra%27s%5d%2b%5bGuardian%5d). Several turns later I equip [Swiftfoot Boots](http://gatherer.wizards.com/Pages/Search/Default.aspx?name=%2b%5bSwiftfoot%20Boots%5d). Does the "hexproof" ability make Serra's Guardian an invalid target for Kasmina's Transmutation?