That is very interesting. Note that they are not precluding you to switch, but they do reserve the right to sell or not sell to you the commercial license after you've used the LGPL. It sounds like an empty threat to me (if you talk to the sales rep and say "I have an internal prototype and I would like to buy the commercial license to finish and distribute it", will they say no?), but one should definitely consider this before choosing to try the LGPL version for early product development.

I think this is mostly meant to avoid having a team of 50 develop an internal version based on LGPL, and then a team of 1 maintain the commercial version, paying a single developer seat.

Note that you can still do a throw away prototype without problems. Some say you should in any case!