Fossilized conversation partners

In Korea (and in most other places too), a thing that happens is that partners in language courses tend to get fossilized, which means that the same people tend to sit together for all classes during the course.  I hate this for a lot of reasons including:

1. That poor person who gets stuck with the worst student in the class.  The burden should be spread among everyone.

2. It gets boring to talk to the same person everyday.

3. It doesn't train students for life.  I want my students to be able to converse with almost anyone, in English.

4. Mistakes get fossilized among partners.  Maybe someone makes a mistake that impedes meaning.  Their partner asks for clarification once and the person gives it and then continues to make that same mistake over and over and never gets any more feedback that that mistake is impeding meaning.

5. There's no chance for many students to encounter a partner at a slightly higher level of language development (the zone of proximal development), which can be extremely helpful.

It's really easy to mix it up and make the students change partners.  I usually do it randomly by just assigning numbers or letters or whatever, but there are plenty more scientific ways to do it too.  I teach the same class twice a week, so I'll generally let them sit with their friend for one class and then assign a random partner for the next one.