cpach

To be honest, I wouldn't buy a fork with the plans of replacing the spring unless you have an exceptionally good idea of what you're doing. I haven't done as much extensive work on Fox forks (damn you Covid, was scheduled for a class that got canceled) but to my remembrance they mostly change travel with new air shafts, and I think they may not have fully upgrade able spring assemblies. Fox is really helpful so they may tell you exactly what you need if you contact them.

Also, I think you're giving the air spring a little too much credit. Better ones are typically a bit more supple of the top, but a very significant difference between forks is damper design and complexity. Rhythm with Grip is honestly pretty good as it is, though.

Rockshox does pretty well with this--if you happened to find a deal on, say, an older Yari you can pretty much drop in the most recent air spring and damper for a Pike and have what is in most respects a 2021 Pike, but this isn't the best plan economically most of the time (unless you happen to already have the Yari).

NONE of this makes any sane economic sense if you have to pay anyone else to do pretty much any of the work. You'll need to invest at least a little in appropriate tooling to do any of this, although it's not necessarily that bad depending on the exact fork. Should probably get the right dust seal press, some good circlip pliars, maybe need a damper bleed kit, some Knipex flat pliars are useful for many of the thin, flat wrench surfaces if you don't have specific tools, etc.

Keep in mind suspension components, particularly on mountain bikes, are pretty significant wear components, so this is all insane also if there's like any stanchion or bushing wear.