Altering texture instead of covering it with new texture?

[LudiusMaximus]

I have a frame with a texture that is defined in xml:

<CheckButton name="BagnonReagentbankToggleTemplate" inherits="BagnonMenuCheckButtonTemplate" virtual="true">
  <Layers>
    <Layer level="BACKGROUND">
      <Texture file="Interface\Icons\Achievement_GuildPerk_BountifulBags"/>
    </Layer>
  </Layers>
</CheckButton>

Under certain circumstances, I want to change this texture in my lua code:

function ReagentbankToggle:Update()
  if (not IsReagentBankUnlocked()) then
    local tex = self:CreateTexture(nil, "OVERLAY")
    tex:SetAllPoints()
    tex:SetTexture('Interface/Icons/Achievement_GuildPerk_BountifulBags');
    tex:SetVertexColor(1,0.1,0.1)
  end
end

But this solution always draws a new Texture on top of the others. With \fstack I can see them becoming more and more with every Update(). Isn't there a way to just alter the already existing texture? Or at least delete the old one before creating the new one?

My problem might just be that I don't know how to get a handle for the old texture into a variable...