To upgrade a vinyl liner pool to a custom concrete pool it is necessary to remove the existing pool and replace it. If the shape of the pool is what you desire, the same excavated hole can be used.

An autocover is a good way to retain heat in the pool. Autocovers are good for safety and for keeping the pool clean. A solar blanket, a bubble cover, and thermal blankets are used to retain heat, however, they are a bit cumbersome. Solar panels can be used.

It's best to consult a professional for your specific needs!

Solar pool heating systems are definitely effective. The Sunstar brand (by Magen) typically will heat your pool 15 degrees warmer throughout the year, so depending on your state/weather, you can have a decent swimming season vs. an unheated pool. It is also significantly less expensive than gas heaters or electric pumps, due to out of pocket initial cost plus ongoing monthly electricity and gas costs for those. Once a solar system is installed, it should have relatively no maintenance costs. Our website has a break-down for you on how they work and comparison to other heating options:
http://www.univ-solar.com/solar-pool-heating-systems/