Can't I just use the Coin GPL Edition to develop my software in-house, and then get a PEL and switch to a non-Free license just before I release it?

From a strictly legal point of view, this is possible. As long as you do not release binaries, you do not need to release the source code either, so you can work on a piece of software under the terms of the GPL, and then re-license it before releasing it.

We strongly discourage this approach, though, for several reasons. First, the Free Edition of Coin is intended for Free Software developers, and using it for proprietary development obviously is against this intention.

Second, the PEL is meant to be a per-developer license, taking into account that you are likely to need professional support when doing your development. By getting a PEL when you start developing your software, you can benefit from our priority support when you actually need it, during development.

Third, note that even if you never release the in-house GPL version to the public, the terms of the GPL are valid for all the people involved in the project. That means that everybody with access to the software has the irrevokable right to get the source code and distribute it to whomever they want, including posting it on public websites etc. This would be perfectly legal. This is the freedom granted by the GPL, and it intentionally clashes with proprietary development.