A quick conversion tool would imply that switching from global variables to functional global variables are better (e.g. reduce code overhead, race conditions) when they are not specifically. Functional global variables can be expanded into action engines which can be more useful but a conversion tool wouldn't be appropriate for this.
As a tool to refactor inherited code it would be nice if LabVIEW had the ability to convert global variables into functional global variables. I did a search and did not find this idea and will be surprised if it is not a duplicate.
You would pop up on the global and select "Convert To Functional Global". The vi includes error terminals in the standard locations but are (obviously) left unwired. The name of the FG vi would default to a concatenation of the name of the global vi and name of variable.
I will leave it up to the experts to come up with a better icon.