Fairly easy, but more work than you'd think, especially on a tidy road bike.

From an mc21 you'll need;
Loom inc everything needed for the GPS
PGMIII
Clocks
Clock mount
Flywheel
Top yoke
L/H switch gear
Ignition switch
Petrol cap
Helmet lock (l/h lower fairing mount)
Lock mech from tail cover.
Anything else needed for the steering lock.

Remove the following from the mc28;
PGMIV
Entire wireing harness
Clocks
Clock mount
Flywheel
L/H switchgear
Top yoke
Steering lock mech (forks out, probably oil res out as well just to get there)
Petrol cap
Tail cover lock mech
Helmet lock

Install the bits from an MC21 instead, I think that's about it.

Things to note (apart from the obvious, i.e. no smart card, no fancy dash etc.);
A PGMIII is not located where a PGMIV is, it's attached to the rear subframe. I have no idea if it will happliy sit where it should with all o.e. 28 bodywork and metalwork in place.
Leave the mc21 flywheel out at your peril, an mc28 flywheeled engine on a PGMIII could well be the worst combination of all.
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