Hi,
Using the power head with a clear flex hose to pump the water from your 32g container to the tank, will save your back.
I remove a little at a time (3x 5g buckets at a time) and refill the tank, then repeat until I have used all the change water. I always save the last bucket of water I remove until I have completed the change...just in case I removed too much water.
Some folks are more secure and remove all they intend to remove, and then fill with new water, I messed up my first try, and removed more water then I could replace, and had to get more from the LFS...since then I do it the way I mentioned above. Anyway I just thought I would toss it out there so you can be aware of the mistake I made, giving you a heads up so you don't what I did.