I'm not sure i understand your question exactly but if you could set up alternate items.

http://help-sage100.na.sage.com/2019/#Subsystems/IM/IMMAIN/INVENTORY_MAINT_ALT_OVERVIEW.htm?Highlight=alternate%20item

http://help-sage100.na.sage.com/2019/#Subsystems/CI/CI_ComEntry/ALTERNATE_ITEM_SELECTION_OVR.htm?Highlight=alternate%20item

If you set the old item Product Type to Discontinued and set up an Alternate Item, then in Sales Order Entry, if you enter the old item code, you will get a prompt warning you that the item is discontinued and it will have three options, Yes to continue using the old item code, No to not use the old item code (clears the field), and Alternate which will display a list of alternate items to pick from.

Now if you mean you'd like to mark an item as inactive yet have it point to a replacement item so if a user in Sales Order Entry enters the old item, it will automatically fill in with the new item, then that is a little trickier to accomplish, you could use a general alias set up on the new item using the old item code BUT, you would have to renumber the old item so it is no longer the same as the code used for the alias. If you don't want to renumber the old item, you can still use the alias but the user would have to click the Alias Item lookup and could then search to see if the item code they are trying to enter has been pointed to a new item. This would work if you mark the old item as inactive that way the user will get a prompt saying the item is inactive and can't be used, nudging them to use the Alias Item lookup. 

I'm sure you could also get creative with scripting to make a better solution but this is all i can think of off the top of my head with standard features.