Since the mezuzot were affixed when there was a door, and they have not been moved from the doorframe, you are not required to remove and reattach them.
 Agur B’ohalecha 16:28:67. He adds that this seems to be the custom. Sha’arei HaMezuzah (11:44) states that this is the majority position. Cf. Pitchei Teshuvah 286:13 and sources cited in Sha’arei HaMezuzah 11:43:96.