can I replace CI ion source with EI ion source to get the same operation (of EI ion source)

A full set of CI capable instrument is far more expansive than an EI capability only, So you have a CI installed instrument, congratulations, From Some 5973N and on our CI instrument had given an option to run EI on a CI source, hopefully your instrument is new enough, the idea is even we run daily experiments on CI , still occasionally EI is needed, so Factory design an alternative, after you tuned the PCI tune and passed, you can evoke to generate a EI_CIsource.U tuning file, this means Software will generate a feasible file to run like EI on an CI source, Please, try it!  

EI on the CI source, well, frankly, does not work well at all. You can get spectra, but they will not be full EI spectra as the tightness of the ionization volume leads to weird ion/ion interactions and other strangeness.  It is very much not recommended. It might work, but you are on your own trying it.  

EI on the CI source, well, frankly, does not work well at all. You can get spectra, but they will not be full EI spectra as the tightness of the ionization volume leads to weird ion/ion interactions and other strangeness.  It is very much not recommended. It might work, but you are on your own trying it.