Minuteman said:....But at the same time, unless you have a dinosaur panel (FPE, Zinsco, Wadsworth, Push-o-Matic, etc) than why not just push three #4's and a #8 and drop a 100-amp in the the new panel and be done with it? 310.15(b)(6)
Just make sure the neutrals & grounds are properly seperated in the existing panel, and the bonding method is removed as well.