Q. A married couple converted to Judaism and the tevila was done on the same day prior to their chupa. What names should be written in the kesuva, the old names that they used until now or the new names given to them by the Bais Din at the tevila on that same day?
A. See question 935 in this forum. Minchas Yitzchok (136), Hanesuim Kehilchosom (16: 47) and others quote Teshuvas Horosh (15: 14) that rules that if the new name given to the ger was not yet established the kesuva would be invalid. However, Shevet Halevi (8: 270, see also Kesuvo Kehilchoso p. 304) maintains that in our kesuvos, since they are enforced by witnesses and the document itself serves only as a proof, even if the name was not established for thirty days, after the fact, the kesuva would be accepted. Tel Talpiyos (Tishrei 67) in an article by Horav A. Broyer mentions that a Kesuvo is different than a get, and if the ger agrees and accepts that this is his new recognized name, no thirty days are necessary.
Horav Moshe M. Lowy Shlit'a, as well as other Batei Din maintain, that if the name was given at the time of the gerus, in the presence of a Bais Din and a K'sav Gerus document was signed then, it suffices for the establishment of the new Jewish name and it should be used on the onset in the kesuva.
Horav Shlomo Miller's Shlit”a opinion is that the complete new given Jewish name should be the name written in the kesuva, followed by the term “demiskeri” or “demiskaryo” (named) and then the old established name they used before conversion. (See also question 936 in regards to the Rov's opinion on a Chupa done in the same day as the gerus)
Rabbi A. Bartfeld as revised by Horav Shlomo Miller Shlit”a