1. Change the time signature in the Time Signature tool, making sure that rebar is deselected. Make sure you don't have any overfilled measures.

2. Go to Utilities/Change/Note Durations... and select change all note durations to 200% or their original value. Include tuplets but, to be on the safe side, deselect Rebar music.

Note that empty measures will not double in value unless you add explicit rests to those measures, which will be a bit more tricky in triple time than in duple time.

I strongly suggest you do these manipulations on a copy of your file.