There is no simple answer. Welding qualifications cover such a wide spectrum of industry, which is bound by different legislation, standards, practices that a specific answer cannot be given.
Depend on what code/directive/pecificationn that was orginally used the qualifications of the welders may be similiar, though some testing allows the use of power tools to clean and prep welds between passes other test do not. Having said that, if the welders need to qualified to ASME codes then a retest is required.
Vita sine litteris mors est.