Just to clarify you’re all going to issue an EIC for a board change even if the installation doesn’t comply with BS7671 but your work at the board does?
If the board install complies, then you can issue an EIC for
your work (or explain the departures from 7671 if you believe you are reasonable to do so). It's just a case of ensuring that the extent of installation box is clearly defined what you did and didn't do to avoid issues in the future.
Most issues where the an installation doesn't comply with 7671 to an extent that affects safety would be resolved by a correctly done board change (RCD protection, correctly chosen MCBs, etc.)
Were there any C1 conditions, then I'd not re-energise them unless they were resolved, but I'd make sure the client knew that before I started.
There are some other 'critical' parts, such as main bonding, for example, that would also normally be upgraded or verified before a board was changed.
Anything else, like non IP rated lights in Zone 2 might not be, but having them on an RCD would clearly add safety in any case.
It will usually not be possible to confirm cable routes for a board change, but then most EICRs don't either.
The mantra I work to mostly is to leave things safer than when I arrived.
Having said that, if a new board is being replaced urgently (for example because of a melted terminal), then there might well be non compliances that were not corrected, but the customer should be made aware of them.
Where time permits, I usually do an EICR in effect (or the testing for one at least) before I do a board change anyway, because it helps me identify potential causes for unwanted RCD tripping.
So tl;dr: It might be possible for an EIC for a board change to be correctly issued, but there still to be C2s at the property, C1s are probably much less likely.