Hi guys...hey Tom..not to be a smarty..I swear. But in my UPC book measure for 3" pipe is 8 feet. I quote almost exclusive to the UPC book ...but always preface with the fact that codes are specific to each area.
And RTN...yup, Tom's right. Depending on what code system in place where you live...you probably do not have to revent the toilet
Check with local building department if you are not sure which codes prevail in your area...and then start looking at the distance from new toilet placement back to the plumbing vent for the old toilet.
Good luck and keep us posted as you go!!