You can simply install your skin twice, and make 1 skin have old postbit, and new one the new postbit
Say you want your template to have the new postbit, you edit template legacy-postbit (or osmething like that) and make it exactly the same as the new postbit template
Therfor that skin only has the new postbit, since both templates are identicle
Just name the skins with hints that the postbit is legacy or not
Hope that helps