I can't because it's Joomla that created the SEF urls in the first place and they're not stored in the .htaccess file. I needed to add some redirects as the url has now changed and Google would have cached them.
It sounds like you just need a regular redirect. The URL has changed, so you want to redirect from the old to the new for SEO. (It's unclear what you mean by "rewrite existing rewritten url" - as @closetnoc mentioned in comments, you would ideally "correct" the first rewrite in this case. But the SEF URLs in Joomla are not "rewrites" in this sense.)
Any redirect you implement in
.htaccess will need to go before the Joomla front-controller.
It's not clear how "generic" you need this to be (how many URLs are we dealing with?), but to literally redirect the URL as stated, you could do something like:
RewriteRule ^boats-for-sale/fairline-squadron-55/1280 /brokerage/boats-for-sale/boat/1280=-fairline-squadron-55 [R,L]
=- in the substitution does look a bit out of place?)
For a more generic solution, maybe something like:
RewriteRule ^(boats-for-sale)/([^/]+)/(\d+) /brokerage/$1/boat/$3=-$2 [R,L]
This will redirect the above URL and any URL of the form
R (temporary 302) to
R=301 (permanent) only when you have confirmed it's working OK - to avoid caching issues.