I don't see any IFRS provision to prevent you from adopting actuarial valuation at any point.
Note that IAS 19.59 says "this Standard encourages, but does not require, an entity to involve a qualified actuary in the measurement of all material post-employment benefit obligations. For practical reasons, an entity may request a qualified actuary to carry out a detailed valuation of the obligation before the end of the reporting period. Nevertheless, the results of that valuation are updated for any material transactions and other material changes in circumstances (including changes in market prices and interest rates) up to the end of the reporting period."
