Dear Rabbi, My father Matzeive in 50 years is beginning to slowly crumble, the weather, erosion and acidic rain is working. The quality of the tombstone on the beginning was not the best, I would like to change the existing concrete cover plate to marble one (no change on head stone with writing). Is there any restriction to uncover the place of resting for short period of time and do I have to make a new unveiling?

Erecting a Matzeiva is an obligation as we mentioned on question 2728. “Yaakov Avinu erected a monument on Rochel’s grave (Bereishis 35: 20). Mishna (Shekalim 2: 5) establishes that obligation and payment dues, to erect a tombstone as asserted by the Tur, Shulchan Aruch (Y. D. 348: 2) and Teshubas Horashbo (56). The Zohar, the Arizal and Sefer Chassidim (738) also place great importance and significance on the monument named Nefesh or soul.(See Gesher Hachaim, Kol Bo and others). However, regarding the ceremony, Nitei Gavriel (Avelus 2: 67: 3) writes that “after researching all Seforim dealing with the traditions of communities, chevros kadishos, avelus and related topics, he could not find a source for the order of the ceremony of Hakomas Hamatzeiva.” He mentions, what is actually said and recited, depends on each community and the Sidurim and similar texts they use.” Therefore, you should properly fix the Matzeivos as needed. The existing concrete cover would likely be broken into pieces and those should be buried underneath or on the side of the new marble cover plate. There is no restriction to uncover the place of resting of your dear ones, for the short period of time needed for the repairs. Although, as mentioned no ceremony is basically needed, you should dedicate some time for learning Torah and helping Torah institutions in the name of your dear ones.