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Does this come into play when replacing SERFs on a board to plug in MCBs, no alteration of base

I thought about this a bit and thought no, because it's simply a replacement of a fitting with another and can be done outside the requirements of 3000

Just curious if I'm way off on on the right track

Good question.
Replacing a SERF with an mcb is replacing a fitting.
The fact it can be done without disconnecting or reconnecting anything doesn't stop it being PEW (as defined in Schedule 1 of ESRs)
and I believe it's outside what ESR 57 permits a homeowner to do.
True "removing and replacing fuse links" is allowed, but that's replacing a link with another link (of same rating); not changing from a link to to a different sort of device.

Upgrading to mcbs as 'maintenance of an installation' would be low risk PEW, so ESR 59(3) allows it to be done i.a.w. MIs for the mcbs.
That way it wouldn't trigger 2.6.3.4.

I was thinking more 59 (3) b

But the same reasoning being low risk work and not needing to be done in accordance with 3000

Can't be (b), because original condition was SERFs.
But 3(c) option of following MIs also avoids having to follow "3000"

Oh, I think I've gotten mixed up with something else

I thought replacing a fitting with another fitting was low risk and didn't require compliance with 3000 or have I gotten myself confused up with something else

You are on the right track; it is low risk.
And there is a relationship between risk classifications of ESR 6A and what rules we have to follow
But risk classification isn't what tells us whether "3000" (and other relevant Standards) applies; or at least not directly.
"3000" applies always to high risk and general PEW, because they are installation work; covered by ESR 59 (1) & (2)
But maintenance is ESR 59(3); where there are 3 options permitted - one of which is following "3000".