May one continue wearing Shabbos clothing on Motzei Shabbos which is Tisha Beav? Are they to be switched after nightfall before Maariv?
In general, it is forbidden to wear Shabbos clothing during the nine days [with exception to Shabbos], and certainly on Tisha Beav this prohibition applies. Accordingly, some Poskim rule that one is to remove his Shabbos clothing immediately after the conclusion of Shabbos [after reciting Baruch Hamavdil], prior to Maariv. According to this approach, it is proper for Shul’s to schedule for people to go home between Mincha and Maariv, change clothing after Shabbos, and then come to Shul sometime after Shabbos for Maariv and Eicha. However, other Poskim negate the above and rule that one may remain wearing his Shabbos clothing even after Havdalah on Motzei Shabbos. Practically, the widespread custom in many communities on Motzei Shabbos which is Tisha Beav is like the former opinion although some follow the latter approach. The Chabad custom is like the latter opinion, to continue wearing the Shabbos clothes until after Eicha and Kinos, and so was the custom of the Rebbe. The Rebbe spoke quite negatively of the custom to change one’s Shabbos clothing. Those who follow this custom are to wear their Shabbos clothing [i.e. hat, shtreimal, Kapata] before the conclusion of Shabbos, and continue wearing it after Shabbos. According to all one may not change his Shabbos clothing before the conclusion of Shabbos. According to all, leather shoes must be removed immediately after Shabbos, and the above discussion is limited to other Shabbos garments.
 Rama 551:1 “By a Mila the custom is for the Baal Bris to wear Shabbos clothing, although otherwise it is forbidden.”
 Maharil, brought in M”A 559:10 “After the Mila they remove the clothing”; Piskeiy Teshuvos 553:1; Nitei Gavriel 94:5
 Nitei Gavriel ibid
 Ashel Avraham Butchach Mahadurah Tinyana 551
 The reason: As the accustomed prohibition of wearing Shabbos clothing is only the initial wearing, while if he already wore it under allowance, such as on Shabbos or for a Mila, then there is no prohibition to continue wearing it. [Ashel Avraham ibid]
 Nitei Gavriel ibid; Minhagei Mahariv Sulitza [Rav Yaakov Yisrael Rubin 1900’s] 507 that he remained wearing his Shterimal for Maariv and removed it before Eicha; Yechidei Segula in Yerushalayim would remove the Shtreimal after Eicha
 Yoman of Rav Groner that the Rebbe stated that so was the custom of Jewry in Russia, and of Lubavitch Chassidim; Rav Eli Landa confirmed with his brother Rav Y.L. Landa that the custom is to remain wearing Shabbos clothing until after Maariv/Eicha.
 The diary of Rav Leibal Groner documents the Rebbe’s reaction after being told by Rabbi Groner of the custom of some to change their Shabbos clothing “The Rebbe was very surprised at this and asked me to show him where it states such a thing. I showed him in the Sefer Tishe Beav Shechal Beshabbos who brings that a number of Chassidic communities are accustomed to do so. The Rebbe responded that in Yakotrinslov no one did this and I do not understand this and why to delay Maariv for such a thing. Later the Rebbe said on Motzei Tisha Beav that this custom of delaying Maariv to go home and change Shabbos clothing is a wild custom that was the custom of Hungarian Jewry but not in Russia. The Rebbe added that the Rabbannim, and you included were never in Russia. Practically, the Rebbe would remain in his room until after Shabbos was over, would remove his shoes, but not his Shabbos clothing.”
 May one even initially put on the Shabbos clothing after Tzeis if he was not wearing it beforehand? If one removed his Shabbos clothing, such as he removed his Kapata, and it is now after Tzeis, it is questionable as to whether he may even initially put on his Shabbos clothing for Maariv and Eicha, as the entire Heter of the Ashel Avraham ibid was based on the fact one remains in his Shabbos clothing into Shabbos, however to initially put on, who says is allowed? Accordingly, ideally one is to wear the clothing before Tzeis. Nevertheless, even if one took it off and is now going to Shul for Maariv, there is room to learn that he may wear it. This is similar to the Heter written in Shulchan Aruch for a Baal Simcha to wear [put on] Shabbos clothing on Tisha B’av itself [See Rama 559:8; Bigdei Yesha, brought in Kaf Hachaim 559:65], and the reason is because it is only a Minhag not to wear Shabbos clothing on Tisha B’av and not forbidden for the letter of the law. This is why we wear Shabbos clothing on Shabbos even though the same Rama writes its forbidden. The same way the custom allows wearing it on Shabbos Lekavod Shabbos, it allows wearing it even initially on Motzei Shabbos Lekavod Motzei Shabbos. Vetzaruch Iyun!