I wouldn't spend the money to replace it. Doing the cost analysis when I purchased my pool, the system would never pay for itself. It works great in the summer (when I don't need it), and can't heat the pool to the temps I want during the cooler months (I keep my pool at 88-89 during the winter months). The only way to heat the pool at that point is to have an another heat source - either gas or electric.
My electric heater maintains my temp at an average of $30/month ($0 from early spring to late fall, and then $40-$60 during cold months). Using a solar system will take $0-$30 off my cost during the colder months, so at an installed cost of $5K -$8K it will never pay for itself.
Just a thought
P.S. I wasn't going to comment, but you complained that no one was commenting