Is it possible to take an existing object and turn it into a clone of another object? Of course, the right way is to do things in the correct order by creating the original object first and then make clones. But sometimes I realize later in the workflow that I would like certain objects to be modifiable all at once. I wish I could then copy one of the objects, make it my "master template", and then link all the other objects to it, without having to delete them all then making clones and having to place all the new clone onjects.
As far as I know, there's nothing that will do this fully automatically.
However you could semi-automate it.
Clone one of your objects, move the clone off the page, and move the clone source off to the other side of the page, so you know where they are.
Change one of the colours in the clone source object so you can better see the replacement in the later steps.
Select all your objects to replace, and look at the status bar to count how many there are. Take a note of this number
Using the Tiled Clone functionality on the clone, add as many clones as there are objects (as calculated in the previous step).
Select and move all the tiled clones to the top of the stack (using the Raise to Top button or the shortcut Home). The later steps for replacement rely upon the z-order in the stack.
Select all the objects to replace and all the clones together, do not include the clone source in the selection.
In the Align and Distribute panel, under Rearrange, hit the Exchange in Z-order button repeatedly until all the objects are replaced with clones. Note that this could be tedious if you have a lot of clones.
Now the objects are replaced by clones, and if you edit the clone source, you should see everything works. Finally delete the left over objects that aren't clones.
A quick example showing the replacement process
I got this idea from a tutorial on youtube by Unfa which goes into more detail if you get stuck on any of the steps above. The only difference here is that I began with a clone to do the Tiled Clones step.