To worsen Eric's bad news, particularly if this is a parameter that's been around for some time and has data in it, I'd recommend you:
edit a custom form on which the parameter is used
rename the old parameter (e.g. myparameter_old)
save (and close) the form to make that "stick"
reopen the form
create the new parameter (e.g. myparameter) with the correct data type beside the _old parameter
save the form
edit every form that uses the parameter and add the new (myparameter) beside the old (myparameter_old)
create a view that shows both new and old, and group by custom form, by myparameter_old value
for each block of each custom form / myparameter_old value, CAREFULLY bulk update myparameter to match
if you have so much data (or the values are so varied) that this approach takes too long, consider using our Excel Updater solution to save time
once all new values are updated CAREFULLY delete the myparameter_old parameter
confirm all your filters, views, reports, groupings and sorts still work using the new myparameter and adjust them as necessary until they do
TIP: if you are not certain where myparameter is used, consider running our Report all Report Details solution first, which will highlight that impact analaysis in advance (in Excel), which you can then use as a working copy to guide your cross checking and testing
For future reference, it seems to be "the presence of data entered anywhere" that Workfront uses to then deny the ability to change the datatype. In rare (and perhaps dubious) cases, I think I have been able to convert certain combinations (e.g. Numberic to Currency), but not in quite some time, so I am no longer certain.
Good luck with it!
Regards,
Doug
