I would not recommend calling

$UserBasePacletsDirectory = "..."

since I think that would cause breakage.

From the documentation it looks like the correct way to change the \ $UserBasePacletsDirectory is on kernel startup using the option `-pacletbase`. If you are using a standalone kernel (from the command line) you just add `-pacletbase /path/to/dir` and in that kernel session it will be `$UserBasePacletsDirectory` is set equal to that variable.

If you are using a notebook front end, then you would need to go to the menu Evaluation->Kernel Configuration Options and add this option to the kernel launch command.

Personally I don't do that, I leave that variable on the factory setting and add this line to my kernel init.m file: PacletDirectoryLoad["/path/to/dir"] which ensures that any paclet residing in that directory is available to me.