The more versions back you are, the more errors you're going to encounter if you try to play the old save anyway.
So if you're talking reeeally old, yea, it will be pretty unplayable.
On the bright side, you shouldn't need to start 100% from scratch, since the game will fix itself if you can load in far enough to start a New Game Plus (NG+). So at least you can keep your stats, appearance, items, abilities, clothing, and money.
(If you get the error message "Failed to load: img/faces/p-zbrow1.png" there's a way to fix that.)
On a further bright side, if your save is from version r37 or later, you should be able to fix any errors the game will have by going to the island and completing at least 1 battle. (As in, you won't need to start over at all.)