How to convert old Bridge .workspace into currently used .xml?

You can't really, although Bridge does internally convert old workspaces during the first run process.

Hi, @LorenaBG, this may sound tedious, but this is one of those situations where it will be faster, more efficient, and have fewer side issues to just re-create them than to try to convert the old ones to the new ones. 

Several things to be aware of with the new Workspace dynamic. First, Bridge has ONE feature that all other Adobe applications should emulate: you can update an already-made Workspace. This is worth the price of the extra work to re-create

Next, up is it is now much easier to have control on what you see and do not see. On the far right side are a pair of chevrons. If you mouse-down on those you can see all of the Workspaces and an "Edit" option. Click on that.

This brings up this window. 

Everything above where it says "OverflowMenu" will show if Bridge's window is pulled out far enough. Otherwise they will show by mousing-down on the chevrons. But also note the "Do Not Show:" option. If you have any Workspaces that are built into Bridge but you'd never use them, you can dump them into that cubbie hole.

Now, one warning: there is a bug that has not been completely fixed yet where you can design a Workspace 1 with Folder A. Then later, while working with Folder B, change back to Workspace 1 but find yourself looking at Folder A again. This does not always happen, and it's been very hard to replicate this by the engineers so fixing it for good has been a challenge. Nonetheless, not every one is seeing this AND Adobe knows about this and is working on it. 

Enjoy using the new Workspace features.

I am using MacOS and Bridge data is comming from an old Windows laptop.