Upgrading my saltwater tank from a 40 to a 90 Can I do this with little or no problem to my fish?

not a problem

You will be fine.

I'v done 60% water changes all at once many times [ although I don't recomend this}. Just check you salinity in the new and old water to make sure they are the same befor you mix.

A problem you might run into is with the new rock, introducing new inverts to your tank. I got a mantis shrimp once...Geez...took three weeks to catch. They have the force of a 22 cal bullet and can break your tank ! After that I just added cultured rock a piece at a time and let my tank bring it to life.

I am assuming your tank is already established with good test readings.

You will have major problems. The tank will not have time to cycle again. You will have high ammonia and nitrites for up to 6 weeks. I think you should keep your fish in the 40 gallon until the 90 cycles. It should be fine once your ammonia, nitrites, and nitrates die down. After this you can add your fish back using proper acclimation techniques.

There shouldn't be any ill side effects. Just make sure you check your salt levels and every thing before adding your fish. I have successfully changed over to larger tanks several times before with no ill problems arising. Your going to using your filters and every thing from the old tank I gather. As long as you transfer all the old eqiupment over to the new tank you should be fine. Good luck,