You'll need to be running two different versions of GoMule. The new 2.4 version, and the previous version (which for sanities sake I'm going to call 2.3.)
Anything you open in the D2R game will be converted to 2.4 and will not be readable in the 2.3 GoMule. (Likewise, any old 2.3 shared stashes/characters/GoMule stashes can't be read by 2.4 GoMule.)
To convert characters and shared stashes to the newest version you just need to open them in game. (As Snickersnak observed, the shared stash gets "opened" whenever you load a character with access to that stash into the game. The character doesn't have to actually open the stash in game.)
The hard part is GoMule stashes, as the game can't open a GoMule stash to convert the items. So, in order to update the items, they need to be loaded onto a character or shared stash to be converted by the game. In order to move items from a 2.3 GoMule stash, you need a 2.3 character or stash file. Below is basically a wordier version of what Silo posted for the procedure. (Sometimes I find that wording something a bit differently can help clarify stuff as different people's brains work differently.) The real trick is to keep everything's versions strait.
- First, put your current in game shared stash file in a safe place so you don't accidentally overwrite it. (You can probably just temporarily rename it, or move it into a different folder for safe keeping.) If you're paranoid and want to avoid any possible mistakes, you can also just move everything in your 2.4 Shared Stash into a 2.4 GoMule stash (using 2.4 GoMule) so it's empty and safe to be deleted/overwritten.
- In 2.3 GoMule, you load up any 2.3 GoMule stashes and the 2.3 Shared Stash file that Silo provided.
- Move stuff from the 2.3 GoMule stash to the 2.3 Shared Stash.
- Move the 2.3 Shared Stash file into your Saved Games folder.
- Load up a character in game that will have access to that shared stash (so, basically if it's a SC stash, open up a SC character.)
- Now, load the newly converted 2.4 shared stash into 2.4 GoMule, and move that stuff off into a 2.4 GoMule stash.
- You can now delete the Shared Stash file you used to do the transfer.
- If you have more stuff to move, load up a fresh copy of the 2.3 Shared Stash Silo provided and repeat.
- Once you've converted all of your stuff, just restore your original 2.4 shared stash by moving it back or restoring the original name depending on what you did with it for safe keeping. (Or if you just emptied it and deleted/overwrote it, you don't have to anything at this point.)