The piartool -backup command has multiple parameters that allow for specifying a number of archives to copy. For example it is possible to specify a number of archives to backup starting at the current time (-numarch), or each archive number can be specified in a list (-archives). But no parameter will allow specifying a numeric range of older archives to copy, such as from archive #500-1000. It is also not feasible to list out each archive for the -archives command when there is a large number of archives desired to copy (500 501 502 etc.)
If it was possible to specify a range numerically, this would make copying operations for older archives go much more smoothly: "piartool -backup -archives 500-1000"
Another option would be to allow specifying a start time and end time for the archives to copy, such as "piartool -backup -archivestart \*-5y -archiveend \*-2y"
The current workaround is to manually unregister all the target older archives from the primary and then copy them to the secondary, then register the copied archives on the secondary and re-register the original archives on the primary.