I inherited a 16x32 Vinyl In-ground pool and a 6x6' Morgan Spa with this house we purchased in 2014. Previous house had an 8K gallon AGP on BBB, so just had to ramp it up to an 20K IGP. Not really "spa people", we used the Spa once when we first moved in, left it running the winter of 2014/15 ($$) and drained it last winter. Over the Winter the heater froze and broke, flooding the bottom of the control box inside the Spa.
As an Electronics tech, I look at the Electrics for the tub and am not happy with them. House is 20 years old, the tub controller is at least that old technology-wise, as well as rusted and somewhat corroded...it's all mechanical timers and contactors...robust and reliable but old and has been wet.
I have a 1.5HP Hayward Super Pump and S244 sand filter running the main pool. Wouldn't cost that much nor be horribly inconvenient to pump pool water into the Spa and drain back to pool. I'd do it above ground to start with, see if we actually USE the Spa before either replumbing underground or replacing Spa pack.
When I look at replacing the Spa pack, even DIY, I'm about 25-30% of the cost of completely new Spa.
Not a big fan of "lounger" style Spas like this one, I'd rather have a basic style tub where 4-6 adults can comfortably sit.
I realize I would no longer be able to heat the spa and would probably lose the functionality of the blower. Other Cons? A big Pro to me is maintaining ONE pool!