[jaransdell2]

When balancing an equation with H2SO4, when can I change it to H2SO8?
or can I ever?

[hmx9123]

You can't change H₂SO₄ to H₂SO₈ without doing some oxidation, in which case that's not going to be the formula anyway.  I'm assuming that you're trying to balance an equation with sulfuric acid in it.  The only thing you can do with that is to put a coefficient in front of the sulfuric acid, such as in this case:

3 H₂SO₄ + 2 Al(OH)₃ --> 6 H₂O + Al₂(SO₄)₃