How to move your field away from a widely accepted practice that has a catastrophic error?
Two quotes immediately came to mind:

A new scientific truth does not triumph by convincing its opponents and making them see the light, but rather because its opponents eventually die, and a new generation grows up that is familiar with it. - Max Planck
I'm trying to free your mind, Neo. But I can only show you the door. You're the one that has to walk through it. - Morpheus from The Matrix

To elicit a change of the magnitude you indicate, four aspects must be communicated clearly and unequivocally:

The existence of the error (i.e., demonstrate that the prior approach is incorrect). Other academics in the field may not recognize that there is an issue.
The history of the error (i.e., why the prior approach was used and how it has persisted for 50 years). Given its age and prevalence, other academics may understandably presume that the prior approach has been validated before and may dismiss your claims on that basis.
The implications of the error (i.e., what negative impact the prior approach has). The effect of the prior approach may not be clear or other academics may perceive it as negligible relative to the effort necessary to adopt a new approach.
The change itself (i.e., the course of action to rectify the error - the new approach). Other academics may not know what should replace the prior approach.

You should first make sure that these have been satisfied in your previous publications. Assuming this has been achieved, there is really only so much you can do (see the quotes). You obviously can't force people to change their ways. In addition to being patient, you can (where appropriate):

Continue referencing it in future papers and conference presentations.
Highlight it in peer review.
If you teach, incorporate into your courses.
Write a review article of the error in your field (one way to address #2 above).
Communicate (1-4) in relatively plain language on your academic blog (assuming you have one).
Send your paper(s) to the authors of the undergraduate textbooks with a letter explaining (1-4) and why the change should be included in the next edition.
Write your own textbook.