pdfTeX warning: PDF inclusion found PDF version <1.x> but at most version <1.5> allowed

This happens on compilation of the document. I suppose this is due to several .pdfs using a higher version. Here is the the parts of the .log to show which images use a higher pdf-version:

pdfTeX warning: pdflatex (file ./.theme_imgs/TUHH_agenda_icon.pdf): PDF inclusi
on: found PDF version <1.7>, but at most version <1.5> allowed

pdfTeX warning: pdflatex (file ./.theme_imgs/TUHH_logo_rgb.pdf): PDF inclusion:
 found PDF version <1.6>, but at most version <1.5> allowed

Not sure if these warnings should be ignored. A remedy seems to lift the pdf-version using \pdfminorversion in the preamble but this in turn causes

! pdfTeX error (setup): PDF version cannot be changed after data is written to 
the PDF file.

so this either points out another issue within the way the presentation is created, or the fact that I may have misplaced the directive.

Not sure if this could cause issues with other users of the presentation but if adapting the preamble is not desired, maybe one could "downgrade" the respective .pdfs version?