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MOS redistribution CCF #75 was released on July 29, 2010, and I have already done the analysis with version #74, do I need to care?

Only if you are doing spectral analysis of MOS data from observations performed in 2010, or, for reasons difficult for us to figure out, you analysed earlier observations using "cifbuild" with the parameter "observationdate" set to later than January 1, 2010,

A formal error affects the value of the keyword EVALDATE in the header of the MOS redistribution CCF files: "EMOS1_REDIST_0074.CCF" and "EMOS2_REDIST_0074.CCF". This keyword, indicating the end of the CCF constituent validity period, was erroneously set to January 1, 2010. Consequently, the Calibration Index File (CIF) generated by "cifbuild" for observations, whose start date is later than January 1, 2010, incorrectly points to old versions of the redistribution CCF files. This may potentially lead to inaccuracies in the spectral fitting.

Corrected CCF files are publicly available since July 29, 2010: "EMOS1_REDIST_0075.CCF", and "EMOS2_REDIST_0075.CCF", whose EVALDATE keyword value has been amended. Users are recommended to update their CCF repository, regenerate the CIF, and then the MOS redistribution matrices via "rmfgen".

The SOC produced MOS "canned" redistribution matrices are correct, so they don't need to be re-generated, they are available from the calibration web portal:

http://xmm2.esac.esa.int/external/xmm_sw_cal/calib/epic_files.shtml

The Release Note of these new CCF constituents (available at: http://xmm2.esac.esa.int/docs/documents/CAL-SRN-0268-1-1.ps.gz) describes the expected scientific impact of this update.