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You can do the find and replace using phpMyadmin, of course backup the database first.
Open phpMyadmin from your hosting control panel, open the database if required, open the phpbb_posts table.
Click the search tab and then the "find and Replace" button. Type in what to find, the replacement and select post_text column. After you click go it should give you preview before committing.
Be aware find and replace is a sledgehammer, you shouldn't have any issues with unique string like that but less unique text can give undesired results. e.g. find cat and replacing with dog turns catastrophe into dogastrophe. In that case you can put a space before and after but that may not find everything because of punctuation etc.
“Results! Why, man, I have gotten a lot of results! I have found several thousand things that won’t work.”