I have an old version of Lust Doll Plus. Do i have to start over if i get the latest one?
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.)
Too far back?
No you can't save that if you mean 38.1 maybe you can? but you can just set stats preferably your old characters stats into a new one if you wish, just be careful sometimes the game won't even open a save if you updated from a really old version.
Or if you mean 41.0->41.1 it is completely fine, just download and install.