converting multiple objects from rgb to cmyk across multiple pages

I have around 5 files, each with up to 70 pages, and each page has a gray line (outline color gray (30% black) in RGB mode. The printer has asked to convert everything to CMYK mode, , (before creating a pdf). I tried doing a find and replace, but this seems to only find objects on the current page, which does not really help me save time. I need it across the entire 70 page document. Is there any command to do this?

i did a search on the forum and people mention macros or scripts but i have no idea how to use these, so if this is the solution, can someone help me with that?

thanks in advance!

p.s. I know it is possible to convert all to cmyk when creating the pdf but the printer requires ( for whatever their reason) that this be done within the document before i publish to pdf.