Why do this in the first place? What problem does that solve for you?
Often, a change in interface and/or table design can make such a move of data from one table to another unecessary. THere are, just to give one possible alternative, ways to make a record disappear from one portal and appear in the other without actually moving any data, you simply manage either the portal's relationship or a portal filter expression (fileMaker 11 only) to control which portal displays the record.
Portal A maintains records of items in a particular collection or 'collectables'. With Portal B mantaining records of what is either missing, desirable, or needs to be tracked down and found. There are similarirites in the fileds and their data, but not entirely the same. Being an item that is missing, there are some details that are unknown.
The Portal B was an after thought and the database has been working for some time without this function. Hence me first instinct to set variables to get it working at least....
The intention is, that once a Portal B record has been found and 'In' the collection, I would like its record to move over to Portal A. Where the record can be finalised and the remaining details added.
Like I said, there was porbably a better way to achieve this.
The better way is likely not to have separate tables for the second portal. If you put all the data in one table (For records of a "missing" item, those extra fields can be left blank), then you can add a field or two to record the item's status as "missing", "Found", "Desirable" or whatever you need.
Your two portals can use a filter expression set in the portal setup (FileMaker 11 only) or in their relationships so that Portal A does not show the Portal B records and vice versa. "Moving" a record from Portal B to A is then accomplished simply by changing the status field so that Portal B now excludes the record and Portal A does not. If you use a Portal based Filter expression, you'll also need to refresh the window with the "Flush cached join results" option enabled. This can be done with a script trigger set on the status field in your portal.