If I understand you correctly, you have a portal showing several records; in each record there's a IsAvailable field; you want that field to be checked in all portal records?
To do this you need to loop through the portal rows, applying the script to each portal row.
Simple to do:
Go to portal row [Select; First]
Set field ... (as above, in your question)
Go to portal row [Select; Next; Exit after last]
This should change all of the records showing in the portal (but only those showing in the portal, not any that were filtered out by how you set up the portal). Thus, all records that were changed in the portal should show up as changed in the List view.
You may be interested in this:
You're sticking your toe in the water of transactions here, so Todd Geist's series on how to manage them might be useful.