If the transition is organic, the status remains unchanged until the second year when it is achieved. For instance, if an SME grows organically and becomes a large company in year 1, and this status continues into year 2, the company can make an SME claim in year 1, but in year 2, it would need to claim under RDEC, as they no longer meet the SME eligibility criteria.
However, if the change in status is a result of a takeover or sale of a group company, the change is immediate.
An SME being taken over by a large company should consider shortening its accounting period to end just before the takeover. This allows them to benefit from making an SME R&D claim for the period before the takeover. What matters is the company’s status in terms of the SME test thresholds at the end of an accounting period.