I have a 5 yr old heat pump. The compressor is a 12 seer. When it was installed they also installed a new coil but it turns out that it was a 10 seer coil. The system has worked perfectly for 5 years. The technician who tuned up the system today (and discovered the mismatch) said I should replace the coil with a 13 seer coil and he wants $1,000 to do it. Should I do it and if I don't what are the downsides to my existing system.