The only way I've figured out how to do it in the past was as follows:
Go to the directory where your original form resides. Copy the form file (.frm) and paste it into the same directory. Rename the .frm file to whatever you want. Then double click on the file to open up a new project. In the "(NAME)" prompt (within properties window), change the name of the form there to what you just renamed the file too. Save the form (when asked to save the project click cancel).
Back in your old project Go to Project => Add Form then choose existing from the tab of the pop up window. Your new form should be there.
I would like to think that there's a better way to do this, but this is the only way I've found in the past.
Hope it helps.
~Kevin