I'm currently using 3.6.15 and I want to load 4.2.6 but still retain all of the incomplete files and be able to resume downloading them using 4.2.6 after it has successfully installed.
What should I do to achieve this outcome ?
Do I just download the 4.2.6 install file and run it ?
Do I have to backup some files, uninstall 3.6.15, install 4.2.6, then copy back the backup files ?

TIA for any assistance.

Yes this is possible, but you must not uninstall the previous version before upgrading to the newer one.
If you uninstall completely a previous version, some of your settings may not be preserved, and the first time you'll run the fresh installation, you'll see the initial installation wizard.

Note that incomplete files are normally stored in a sibling folder of your existing default download folder. The location of this download folder is saved in your user's settings in the file named "limewire.props", and user's preferences and settings files are in your user's home directory in a ".limewire" subdirectory.

If you uninstall completely Limewire, your user's .limewire directory and its content should not be deleted, but a fresh new reinstallation will not detect the previous installation, and may provide a default location for your preferences. which may not match with the location of your previous installation (in that case you may get TWO .limewire folders on your disk, the old one with its saved settings and specifying the location of the download folder and of the incomplete files folder, and a new .limewire folder specifying by default a new location for the download directory).

Incomplete files should not be deleted by uninstalling Limewire, but you'll have to specify, when reinstalling the new version, where your prefered download directory was, if the LimeWire installer cannot detect it (for example if you have not used the default proposed location for this save directory but specified another one).

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