I might be wrong. but its not possible.
But there is always ways around that. but having several fields with the different types you need. and by clicking the button you can controll what field is showing/hiding on that same spot. that way you get the same function.
Another possibility is a scripttrigger on your field that triggers a script when you try to enter oder modify the field. The script can check wether an input is allowed or not, dependent on a global variable (or a seperate field), that is set by your button.
hmm how do you do that?? Just to make sure, when I mentioned disable I meant that the user cant edit the field but is able to view what was input previous.
If you have not been "properly" Welcomed, then
I might have two fields one for entry and one a "calculation" of the field (or even a Merge field on the layout). Then have a button to toggle (it can even be user specific by accounts' privilege sets). Use "Hide object when" to show one or the other based on the toggle variable or a setting such as the permission.
Here's an example file with 2 Scripttriggers. The advantage is, that the user can still enter the field and copy the content instead of directly being trhown out of the field.
Hmmm,... By reading your last post once again I realize that this is not exactly what your are heading for. I guess what you want is some kind of "history" solution. Please search for "audit logging" on he internet or in this forum.
AllowChange.fmp12.zip 67.3 K