This article considers specifier-to-head reanalyses within the complementizer domain. Such reanalyses are well attested. This article focuses on one such change, the emergence of affirmative main-clause complementizers in Welsh (present-day Welsh mi and fe). It is demonstrated that these emerged from earlier preverbal subject pronouns that satisfied a V2- constraint in [Spec, CP]. The reanalysis proceeded via two stages, one in which the expletive pronoun ef(e), efo was reanalysed as an affirmative main-clause complementizer, the second in which all other pronouns were reanalysed as affirmative main-clause complementizers that showed agreement with a pronominal subject. This agreement was lost between the eighteenth and twentieth centuries, with different results according to dialect. These changes are considered in the context of other cases where new agreeing complementizers have been created in varieties of Dutch and German, and from the general perspective of formal approaches to grammaticalization and unidirectionality of change.