It looks like you may have to recompile the custom jars to be in line with newest jars.
hence I suspect you may have to alter the import statements in your custom jar to refer something like "org.apache.poi.*" instead.
We have around 18 functions with the same library. out of which com.pega.apache.poi referenced in 3 functions, i have modified it to org.apache.poi and tried to compile it but could not save as its throwing the error "cannot be resolved", faced the issue with all other 15 functions as we are using the same library name.
I queried to DB select distinct(pzpackage) from pr_engineclasses where lower(pzpackage) like '%poi%'; and i can able to see the packages for
com/pega/apache/poi and org/apache/poi too.. We are working with GCS team now to resolve the issue.