Let’s get a couple of things straight.
First off an EICR is a report on whether an installation is safe for continued use, not a report to say whether it is safe to make alterations or additions.
If you want to conduct an EICR to check whether it’s safe to change tails and a CU, that’s your decision.
It’s not something required by BS7671.
Secondly an EIC is a record of the work conducted, a record of appropriate inspections and tests, comments on the existing installation (if any), a recommendation as to when the installation should be inspected, a signed confirmation by the designer, installer and inspector that the work complies with the Regulations and a record of when the work was completed.
It’s also required by BS7671, unlike an EICR.