I have a rectangle concrete in-ground pool with an 8ft deep end that I want to reduce the depth to 4.5ft also. There is a main drain on bottom of deep end. I am also trying to figure out the most cost effective quality way to fill in the deep end. How might you do this all the while still allowing the main drain to function properly? The pool is old 1980 and I probably need to have it re-plastered or what ever the other options are at the same time I decide to take the steps of reducing the depth. I am open to any ideas please.

Not a lot of choice with a concrete pool. The drain is easy, elevate it. Filling the pool with concrete is the best way but bonding and extra weight may be an issue. Contact a pool company and let them give you some suggestions. I have seen some concepts of high density poly foam and lightweight concrete.