Depended on which construction code is applied, if ASME Section IX is the one, then preheat decrease >100F, requalification is required. Changed of type of PWHT or no PWHT will also require new PQR. Our practice is to prepare two coupons, one with no preheat (as far as possible and permit by code) with PWHT; another one with preheat(or no preheat ) but without PWHT(if permit by code).
Kind regards
H.Chang