Commentator 'Apryl' on this article found a workaround.
Basically, it consists in using the search feature of your friends list with simple searches. First search using the alphabet one character at a time and then simple syllables (be, na, etc.) as well as sounds and common combos (ch, ng, ck, pl, etc.) then name fragments. Eventually some of these deactivated accounts will come up. When they do click on them and a dialog providing the option to unfriend them will appear.
It's tedious but it works and you may want to consider spending the time if the jump suddenly put you up against the friends limit.
Your friendly neighbourhood webmaster.