Really the deal here is you're interfacing a modern digital instrument to a decades old analog device. Garmin's advertising says that the 275 is "compatible" but I think that's misleading. Compatible to me means it just plugs in. The reality is they need to add additional boxes and do a bunch of wiring and calibration to hook up this "compatible" autopilot. Does it work after all that? Sure. Would it be worth it? Well, IDK. I was fortunate that the shop I dealt with was willing to only charge me the labor they quoted and only made me pay for the additional hardware. Given the amount of time they put into it I think they'd have been ahead to just buy me a new autopilot but hindsight for everyone is 20/20. If you choose to go this route the big thing I'd do is ask the shop specifically if they'd interfaced your type of autopilot with 275 before.
This shop had done lots of GI-275 installs but I was the first one cheap enough not to want to put a new autopilot in with it. They tried really hard to make everything right, I think they really did just quote my install assuming since garmin said "compatible" it wouldn't be that complicated. I had to reach out to POA and other corners of the internet to spoonfeed them what they needed to know which.... you'd think they'd have better info resources than me but I guess not everyone lives on the internet.
Apparently interfacing to a modern garmin autopilot is pretty much just plugging in a connector.