Even if you can update the buff text by changing the string fields of the buff object itself with field natives (which I don't know is even possible), I don't think it would show the updated description on a unit until the buff was removed and reapplied. Such fields for abilities don't update until the ability is set to a different level and then back to the correct level, so I don't see this being any different.
Uncle's solution is probably the easiest, but it really depends on how many different buff text variants you need and how you're applying them.