Hi Karl,
Wow, I didn't even know that Dean put that type of editor in 1.86b! I must have missed that and skipped to the 1.87 version.
You could try copying the file: editmsg.php from your 1.86b folder and putting it into the 1.88 installation folder and see if that does the trick - BUT, you should back everything up, and at least make a copy of the old editmsg.php file first.
There may be some database changes though and it may not work... I don't have that older version, if you send me the PHP file for 1.86b I'd be happy to take a look at it and see where that code is. Email me directly at my website (see below).