You will be able to install it over, but why get Trinity now? Impulse is having a sale at the moment and the SOASE expansion bundle is on sale for 14.75. Much cheaper and won't have to uninstall and all that crap.

I hope I answered your question, even though got a little off-topic, hope you take my advice though!

Hmm, big sale of vanilla Sins a few weeks ago, now a big sale on the expansion bundle?  Not coincidental, I think.

Definitely get the expansion bundle; you'll save yourself money that way.

As for the achievements, there's a single file that stores that info.  Just save the file and replace it after installation and your achievement progress is preserved.