Q.A prior already married couple (to each other) that is now converting and going to the chupa on the same day as the tevila, can they use the newly adopted Hebrew names in the kesuba?
 
A. Remo (E.H. 120: 3) rules in regards to a get, that there is a requirement of thirty days for the establishment of a new name. (See also Shulachan Aruch C.M. 49: 3.) This requirement is essential and it invalidates the Kesuva. (Tshuvas Horosh 15: 4, see also Igrois Moshe E.H. 4 in regards to last minute changes on names due to similarity in the names of the families of the chosson and kallah, Nitey Gavriel – Nissuin 28: 4))
Horav Shlomo Miller’s Shlit’a opinion is that in this particular case the fact that the newly converted couple was given a new name at the conversion ceremony and the name was recorded in the Geirus document suffices, even if that happened earlier in the same day.
 
Rabbi A. Bartfeld as revised by Horav Shlomo Miller Shlit”a