First, I'll point out that Sybase only supports the most recent 2 major releases - 12 and 12.5 in this case (plus the ebfs). Factor that into your decision regarding which version to migrate to. It may be important.
Second, (what I call) gee-whiz code that works in a very old version of PB is likely based on assumptions about how things worked internally with that version of PB. Your current migration involves some major changes in Powerbuilder - it is likely that you will need to dive into this code and migrate it so that it works in the current version if you want support. If your code is really that old (or older), it is likely that it still uses obsolete functions and designs - like setactioncode. It does concern me that you are attempting to manipulate the content of the dropdown list while trying to keep the actual displayed value unchanged. I think you should reconsider this design since it is complicated and fragile - as you are finding. PB functionality has expanded greatly since PB8; there may be things you want to redo entirely to take advantage of them.