Identity is a big confound in the relearning experiments involving foreign adoptees. There are quite a few studies looking at Korean children adopted by French or North American families. These children seem to completely lose their first language (no neural response in fMRI), even though some were adopted as late as 9 years old. However, when they enroll in introductory Korean classes in college, they seem to pick up the phonology more quickly than their classmates.

The confound is that it's not clear if they are "reactivating" some long forgotten part of their brains that maintained Korean phonology, or if they are simply more apt learners due to identifying as Korean. In other words, one of the challenges in learning to pronounce a new language is the ability to overcome "sounding funny" to your peers/yourself, but these Korean adoptees may not have that holding them back since they feel that they are reacquiring a lost part of their identity.