Nearly all countries have a regulatory authority which oversees the production of medical products, those that do not, usually recognise other authorities such as the FDA. In this regulated field there is never a good enough excuse for manufacturing regulated product (including batches for human trials), using none validated equipment, utilities, facilities and or processes.

Prior to an Installation Qualification (IQ) being executed for a piece of equipment, it is required to be comprehensively tested in the vendor’s establishment, by the execution of a Factory Acceptance Test (FAT), this is followed, once on site, by the execution of a Site Acceptance Test (SAT) which is integrated with the site commissioning. You must not be making adjustments or alterations to settings during IQ or OQ, these should all have been completed in the SAT/commissioning phase.

The build history of equipment and software is essential, because during IQ execution, you may have to review it, and assess the equipment / software quality, from it.

Retrospective validation has to be justified, and this justification is usually given in a Validation Plan (VP), which then becomes your authority to validate. In this VP you document your justification, along with the subsequent action you intend to take. You detail the scope and the responsibilities for all the proposed validation activities. You then get the company senior managers to approve it.

Some acceptable reasons for retrospective validation;
Your product was directed at home market, now you want to expand to a FDA controlled market.
Product line was stopped, now you want to restart.
Your company practices and procedures state that all validation will be reviewed at (1,2,3,) yearly intervals.
Legislation has changed.
The original validation was found to be inadequate.
Recommendation from a reviewing consultant has led to your decision to revalidate.

Whatever the reason get it documented in a VP.

Remember validation is completed using a series of documents each one as important as the next one. Get the sequence in place and validation becomes orderly, simple, robust and most importantly compliant.