The newer filters are easier to take apart but they aren't exactly easy. I mostly believe that if it isn't broken don't fix it. If there is nothing wrong with your filter there is no real need to replace it. The savings of doing them both at once won't be all that large.
I'm not really sure why the heater lasted so long, except that we haven't used it a whole lot here in Houston. We've been in the house 8 years and the previous owner was not the best care giver. The pool was replastered/coped/tiled about 4 years ago.
I would love a heat pump that also has cooling abilities, but we have a pool/spa and its my understanding that the heat pumps only go a little above 100k BTU which is not really enough to heat the spa in a reasonable time. It sure would be nice to be able to cool the water below 90 degrees in August. I've read running the pumps at night and using a fountain will help (on dry days) so I'm going to give that a shot this summer.
The amount of time needed to heat up your spa will be proportional to what size heater you currently have. If you have a 400,000 BTU heater, at 70% efficiency (optimistic with a 25 yr old gas heater), you're getting 280,000 BTU output of heat. The AquaCal SQ156, for instance, provides 127,000 BTUs of heat, which is a little less than half the BTU output of a 400,000 gas heater. So, it will take a little more than twice the time it took for the gas heater to get your spa up.
If it took 15 minutes with the 400,000 BTU gas heater, it will take a little over 30 minutes for the heat pump to do it.
That's not too bad or much of a problem. Just switch on the heater 30 minutes before you want to use it, rather than 15 minutes before.
In Houston, the temperatures really don't drop down enough to warrant using the gas and heat pump approach. During the months that you're more inclined to venture outdoors and use your spa, are the same months that a heat pump would be very efficient in maintaining heat to your pool.