I don't think any knife with most locks available are truly swappable. Frame locks, liner locks, compression locks, they all depend on the interface between the tang and lockbar. Swap the blade, you're going to have problems. People swap the blades of para 3s and para 3 lightweights a lot and usually don't seem to have problems, and you cannot swap the lockbars on them. However I think it's luck of the draw. I've seen plenty of people swapping blades on liner and frame locks and have blade play, lock over travel, or no lockup at all.
I got lucky one time. I bent (somehow) my Kershaw Thermite. I bought another one and it had vertical blade play (tang was ground too much). I swapped the blade from my bent one and it locks perfectly.
I think swapping scales and hardware are fine, I've swapped hardware. That's not a big deal imo. Swapping scales can be bad if they are machined badly. Swapping blades? Nah... Too much uncertainty there. Even Hinderers don't get blade swapped, it's just hardware and scales. And there are plenty of scale and hardware options out there for Pm2s. Spyderco themselves don't do it because I don't think Sal likes taking business away from small makers like that (such as with deep carry clips).
Personally? I'm happy with my Pm2s as is. I'm boring though! Have considered Aramis scales for my S30v pm2, but don't think I want to sink 100+ into a knife I paid 100 for and is a fine tool as it is.
Spyderco : Resilience, Tenacious, Persistence, Manix 2 G10, Para 3 G10, Para 3 LW, Paramilitary 2,
BBS Paramilitary 2, Amalgam, Native Chief, Blade HQ Manix 2 XL, Shaman, Gayle Bradley 2, DLC M4 Shaman
Kershaw : Tone, Cryo 2, Thermite, Atmos, Natrix 7007cf, Natrix 7008cf, Bareknuckle 7777, Bareknuckle 20cv, Concierge, 7777CFM390 Bareknuckle
Zero Tolerance : 0566CF, 0770CF, 0770GRYBW, 0562CF, 0470, 0456CF, 0095BW, 0801TI
Ontario : Rat 1
CRKT : Caligo
Civivi : Plethiros, Baklash, Incite, Vexer
Cold Steel : AD10, 4Max Scout, SR1 Lite
Benchmade : 810-1401 Contego, 15080-2 Crooked River, 15080-Custom Crooked River, 560BK-1 Super Freek, 15085 Mini Crooked River, 15085-Custom Mini Crooked River, 940-2, 550-1 Griptilian, 950bk Rift, 940-1, Custom Sheepsfoot Griptilian
Buck : 110, Bucklite 422, 110 LT, 110 Slim Select, 112 Slim Pro, Custom 110, 1978-79 110
Artisan : Tradition
Hogue : Ritter RSK MK1-G2
Kabar : Becker BK2 Campanion