Yes - your old presets stick around. They are tied to the old versions of the instruments, which also stick around. Your DAW is referencing the old AU/VST versions of the instruments - so that all works. You shouldnt have to re-do any existing compositions, unless you want to re-do them with the new instruments.
Where you will hit the wall potentially - is with the presets. You will have to manually convert your saved presets to the newer version of the instruments. In some cases *cough* Jup-8... you may have to completely re-do the presets, because functionality has been dropped (splits and layers dont exist in the Jup-8 V4 - but you can simulate that, by instead re-writing the preset in Analog Lab??). To your question -- moving presets from the old versions to the new is a manual process, that isn't necessarily cut-n-dry nor easy (otherwise Arturia would have provided a converter, and converted all the existing factory patches to the new versions ... which they didnt.
So... imo. The upgraded instruments are a huge step up. Im keeping most of my old Logic Pro projects, and re-doing a few, to take advantage of the better timbres in some of the new instruments. There really is a noticable change (for the better) in the timbres. Well worth re-doing a few channel strips.