There are no hooks into a button that will let you change it via LotusScript or a Formula. If you only have a few different captions you d like to have on a button (and the scripts are the same), I ve once stored those various predefined buttons on a hidden form, in separate richtext items, and then copied them to my doc using LotusScript. That would be useful if you were trying to generate and send a button in an email for example, but not much use unless you re generating the doc upon which the button appears programmatically as well.
You might find useful the undocumented method "addLinkByIDs" on the NotesRichTextItem class. Again, probably only useful if you re building the document on the fly, as changing rich text items on a UI doc isn t very easy to do.
More info on undocumented methods here: http://opendom.blogspot.com/2006/11/undocumented-dom-lotusscript-inventory.html
Lastly, there s always the Midas LSX from GeniiSoft, which is an add-on that gives you lots of control over rich text items. I don t know for sure, but I d bet there s a way to change a button caption using that LSX. http://www.geniisoft.com/showcase.nsf/MidasLSX