How to convert a Forge workspace to a Fabric workspace but in different workspaces

You can copy the workspace folder and then open this second workspace folder in MCreator and switch to fabric.

Then you have two workspaces, one for forge and one for fabric.

I would recommend using one workspace, though, as otherwise you will have to do all work twice

Did you ever find out how to switch your workspace?

I might be a bit late to this topic, but this post is still a top result on google, and I can offer some clarity.

Note: the following instructions are for Windows 10, as that's the only OS I'm familiar with.
 

These instructions should work to port in both directions (Forge --> Fabric, and Fabric --> Forge).
 

Expanding on @Klemen's suggestion:

1)
First, go to your "MCreatorWorkspaces" folder on your computer.

If you don't know where that is the default file path is:
C:\Users\User\MCreatorWorkspaces

Note: There are 2 MCreator related folders in at this location. you want the one named: "MCreatorWorkspaces", and not: ".mcreator"

Alternatively, you can open the project you want to port over in MCreator, At the top of the MCreator window, go to:

Workspace > open workspace folder > then in the file explorer window, in the file path bar, click "MCreatorWorkspaces" to view all of your workspace folders.

Note: you may need to enable "Hidden items" for you File explorer.
This setting can be found in the View tab of the file explorer window.

2)
Once you have your "MCreatorWorkspaces" folder open, create a copy of the project folder you want to port over 
(select folder, Ctrl+C, Ctrl+V), and rename the new copy to something appropriate. 
(I would recommend including the modloader name in parenthesis, so you can easily tell the two apart.)

3)
Now it's finally time to change the modloader of your copied project.

Open MCreator and open your copied project (which should be listed when you open the application. if not, close all MCreator windows you have open, and open MCreator again).
 

At the top of the MCreator window, go to:
Workspace > Workspace Settings > Basic Settings > minecraft version (generator), and click the blue pencil icon to edit.

At the top of the window that opens, where it says "Select generator:" use the dropdown menu to select the game/modloader version you want to use.

(for example, my MCreator lists: "Fabric 1.20.1", Forge 1.19.4" and "Forge 1.20.1" as options.)
 

Note: this window also lists the compatibility of your project with the selected modloader. 
Be aware that Fabric mods do not have access to all the same features that Forge mods have.
I don't have advice on this subject, it's just good to be aware of.

Hello, I've followed your explanation but on "Selected Generator" only shows Forge 1.20.1 and Forge 1.19.4. Does that mean my mod is using features that are not compatible with Fabric? Or I'm missing one step?

Am I*

I think you might not have the fabric plugin installed because otherwise it would show up

@KarKomma404 Yes, @Lcs128X is correct. MCreator does not natively support the Fabric modloader. 
The "Fabric Generator" plugin is required to build mods for Fabric.

Plugin Link

Note: The Fabric Generator plugin is (at time of writing) no longer supported, and many features available for Forge mods are not available for Fabric.

Apologies for the late response.