Do I have to remove the old insulation in the attic before I put I spray the new insulation on?

Do you mean spray insulation or blown in insulation? Spray insulation is only done by professionals. Blown in insulation can be applied over old insulation.

First off you need to know this, being your house was built before 1978 that insulation could have per-lite which is a asbestos additive was put in with insulation, Now they have machines you can rent that will remove all that insulation and blow it into a dumpster. check the web we use these machine when we have a house that had a fire and we have to remove the smoke damaged insulation and replace it with new. also you may want to check the new EPA Laws that are coming out on April 22, 2010 you will need to make sure theres no lead base paints too.

I would say no. The more insulation the better. Insulation doesn't really wear out or degrade with time. Adding addtional would be fine and give your a higher R factor.

YES.....the spray in insulation will not stick to anything if you don't.

no the more insulation you have up there, the better insulated it will be.