A term describing an erosive process of bilingual proficiency

One technical term describing an erosive process of a person’s bilingual proficiency that refers to the gradual subtraction of a person’s L1 as time goes by, and simultaneously her L2 proficiency is limitedly developed, though the adult has lived in the second language context for a long period of time.

(Cf. NotSubtractive bilingualism which is the perception that the acquisition of L2 would be detrimental to an individual’s L1.)