As utnik says, you illustrate a reason to use Paragraph and Character Styles. This would still be a solution -- create a Paragraph/Character Style using Tacoma, then use Story Editor to apply this where needed.
If you don't mind using a plain text editor, you could save your document, then open in a text editor (not Scribus), and do a Search/Replace for Arial, replacing with Tacoma (or maybe Arial Italic, replacing with Tacoma Italic). Save under a different name at first, just to be safe.