At DESY several cavity strings for modules of the FLASH accelerator and studies on XFEL prototypes are completed. In some modules cavities have been exchanged for upgrade of applicable module gradients or exchange of resonators showing performance degradation in the module test. Assembly and repair sequences and quality control plans for cavity string are developed and applied. From module string PXFEL3 six out of eight cavities are removed and handed over to CEA Saclay for training of string and module assembly. On two of these cavities the procedures for assembly and exchange of cavities in cavity strings are crosschecked. These cavities are tested at 2 K as removed from string without additional treatments to cross check assembly and repair sequences.